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Keep the displayed price and checkout amount aligned by deriving both from one shared pricing source—but use different rounding rules for charges and credits. In Daniel Pertu’s account of the Munchable Premium implementation, converted list prices receive a 4% buffer and round up to retail-style endings, while reward credits use the plain mid-market rate and round down. That split reflects a product-specific policy, not a universal rule for pricing or tax.
Why use one package for prices shown and prices charged?
A pricing page and a checkout flow can drift if each implements currency conversion and rounding separately. Pertu describes a TypeScript package at packages/pricing as the shared source used by both a Next.js marketing site and an Expo app; his article says 16 files import it. The package has zero runtime dependencies, according to his account.
The design goal is consistency: the same pricing logic can supply amounts to the customer-facing interface and the payment flow rather than relying on separate, potentially divergent calculations. The article describes Stripe Checkout as part of the implementation, but does not establish that this architecture or its rounding policy is appropriate for every product.
Pertu’s article is the source for the implementation details and figures below. They describe the author’s project settings; they are not independently tested or a statement of current Munchable prices. Read Daniel Pertu’s article on DEV Community.
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How does the package calculate a displayed list price?
Start with a fixed base and fixed local prices
The example base price is £10 per month, represented internally as 1,000 pence. Pertu says the product uses fixed local prices for each currency rather than converting a live exchange rate during checkout; €12.99 is his example of a fixed euro-area price. This separates a product’s chosen local price from whatever a live currency conversion would produce at a particular moment.
Apply the buffer, then round upward
For converted list prices, the described sequence is to apply a 1.04 multiplier—a 4% buffer—and then round up to a retail-style ending. In currencies displayed with decimal places, the price rounds up to the next .99 ending. For large prices in currencies displayed without decimals, Pertu describes rounding to a scaled step, with examples including ¥2,300 and 4,500 Ft. These are examples of his implementation, not a universal formula for those currencies.
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The buffer is a choice made for this product. Pertu relates it to payment-processing conversion costs, but the article’s stated fee range is not independently verified here and should not be treated as a current Stripe fee schedule or as a general recommendation. A business setting its own buffer needs to account for its actual costs, pricing strategy, and applicable rules.
Why do discounts round down instead?
A charge and a credit move in opposite directions: rounding a customer’s charge upward can raise the amount collected, while rounding a promised discount upward can increase the value owed to the customer. Pertu therefore describes converting discounts at the plain mid-market rate, without the 4% buffer, and rounding down. In his words, “a rounding helper is not reusable across money that flows in and money that flows out.”
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His example is a 20-pence reward credit: £0.20 in London and €0.23 in Dublin. Applying the list-price rounding rule to that small discount could produce €0.99 instead, overstating the credit. The rationale is the author’s product policy: rounding a discount upward could create a promise the business may not be able to honor. It is not presented as a formal accounting rule.
How do zero and processor amount units affect the calculation?
Preserve zero
The package passes zero through unchanged. Without an explicit zero check, a general “round up to a retail ending” rule could turn a free price into a nonzero amount.
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Separate display precision from payment units
A currency’s displayed decimal places and the unit expected by a payment processor are related but not interchangeable concepts. Pertu reports explicit handling for Stripe’s zero-decimal currencies: BIF, CLP, DJF, GNF, JPY, KMF, KRW, MGA, PYG, RWF, VND, VUV, XAF, XOF, and XPF. He separately lists HUF, ISK, TWD, and UGX as currencies whose Stripe amounts must divide evenly by 100 despite being displayed without decimals. His example represents a displayed 4,500 Ft amount as a Stripe unit amount of 450,000.
These currency and processor-unit details are reported in Pertu’s article and have not been independently checked against current Stripe documentation. Verify the processor’s current rules for each currency and amount before implementing them; do not infer the required API unit solely from how a price appears to a customer.
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What does this approach mean for tax-inclusive prices?
Pertu says the listed prices in his example include tax, with VAT coming out of the displayed amount. That is a separate pricing decision from currency conversion and rounding: a shared package can keep a chosen price calculation consistent, but it does not determine how tax must be treated in a jurisdiction.
UK VAT guidance distinguishes the treatment of unconditional discounts from prompt-payment or contingent discounts, and rounding can differ depending on whether calculations concern invoice traders or retailers. Those distinctions make it unsafe to generalize this project’s policy to every tax calculation or jurisdiction. For UK tax use, consult current official HMRC guidance; the linked mirror identifies itself as a VAT Notice 700 guidance change and should be checked against GOV.UK: VAT Notice 700 guidance mirror.
When is this design useful—and what must be decided separately?
The approach is useful when several interfaces need to present the same product prices and when customer charges and customer credits need different rounding behavior. The package centralizes implementation; it does not make the underlying policy universal or remove the need to validate processor and tax requirements.
The Tool Desk
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- Charge rounding: set any buffer and retail-price endings based on the product’s actual costs and policy.
- Credit rounding: choose a rule that does not overstate the value promised to a customer.
- Currency handling: distinguish display precision from processor-required amount units, and verify current processor rules.
- Tax presentation: decide whether prices include tax or add it separately, using rules applicable to the customer and transaction.
The practical boundary is simple: share the source of pricing logic so displayed and charged prices stay aligned, but do not reuse a charge-rounding helper for money credited back to customers.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
