Spree::FixedAmtDiscount

Spree's built-in FlatRate calculator doesn't handle fixed-amount discounts well once tax is involved. Attached to a per-line-item promotion adjustment, it applies the full amount to every line item — a £10 promo on a three-item basket discounts £30, not £10. Attached to a whole-order adjustment instead, it lands as a single order-level adjustment that can't be apportioned between tax categories, so a VAT-inclusive store ends up with the wrong tax breakdown. This gem adds a calculator that spreads one fixed amount across the basket's line items in proportion to their value, so each item's share — and its tax — comes out correct.

Requires Spree >= 5.6 and Ruby >= 3.3.

bundle add spree-fixed_amt_discount

In the Spree admin: Promotion → Actions → Create per-line-item adjustment, then choose the Fixed Amount (spread across line items) calculator and set its amount and currency.

• The discount is split across actionable line items weighted by price × quantity, calculated before any discounts are applied — so when this promotion stacks with another one, the split is still based on each item's original price, not its already-discounted price.
• Because the split is proportional, the shares are rounded to the currency's minor unit and won't always sum exactly to the target amount. The last item (ordered by id) absorbs whatever rounding remainder is left, so the shares usually sum to the full discount. In rare baskets where the rounding can't divide cleanly, the applied total can fall a few pence short of the configured amount — it will never go over, and a line item will never be surcharged.
• If the discount amount is larger than the actionable total, it clamps to that total — the order can be discounted to zero but never goes negative.
• Line items excluded by the promotion's rules (e.g. per-item rules or promotion-category restrictions) are left out of the split entirely. Their value isn't counted in the denominator, so they don't dilute the discount shared among the remaining eligible items.

A £10 discount across three line items worth £33.33, £33.33 and £33.34 splits as £3.33 / £3.33 / £3.34 — the third item picks up the extra penny so the shares sum to exactly £10.00.

bundle install
bundle exec rake test_app
bundle exec rspec

The gem is available as open source under the terms of the MIT Licence.

Everyone interacting in this project's codebases, issue trackers, and communication channels is expected to follow the code of conduct.