Every invoice flexprice generates follows the same deterministic sequence. Getting this order right matters when you combine coupons at multiple levels, wallet credits, and tax rates.
The sequence
1. Subtotal
= sum of all line item amounts (usage charges + flat fees)
2. Line-item coupon discounts
Each targeted line item's amount is reduced individually.
3. Invoice-level (subscription) coupon discounts
Applied to the running subtotal in association order.
Each coupon sees the total left after the previous coupon.
4. Prepaid wallet credits
Deducted after all coupon discounts.
5. Taxable amount
= MAX(subtotal - all_coupon_discounts, 0)
Floored at zero. Coupons cannot create a negative taxable base.
6. Tax
= sum of all applicable tax rates x taxable_amount
Each rate is applied independently to the same taxable amount.
7. Invoice total
= subtotal - total_coupon_discounts - wallet_credits_applied + tax
Worked example
| Line item | Amount |
|---|
| Usage charge (API calls) | $80.00 |
| Flat monthly fee | $20.00 |
| Subtotal | $100.00 |
| Discount step | Change | Running total |
|---|
| Line-item coupon: 25% off API calls | -$20.00 | $80.00 |
| Subscription coupon: $10 off | -$10.00 | $70.00 |
| Prepaid wallet credits | -$5.00 | $65.00 owed |
| Tax step | Amount |
|---|
| Taxable amount (subtotal - coupon discounts only) | $70.00 |
| Tax at 8.25% | +$5.78 |
| Invoice total | $70.78 |
Wallet credits reduce the amount owed but do not reduce the taxable base. Tax is calculated on subtotal - coupon_discounts, not on subtotal - coupon_discounts - wallet_credits.
Coupon chaining
Multiple subscription-level coupons apply sequentially. Each coupon sees the amount remaining after the previous one, not the original subtotal.
Subtotal: $100.00
Coupon A (10%): - $10.00 --> running total: $90.00
Coupon B (10%): - $9.00 --> running total: $81.00
(applies to $90, not $100)
Line-item vs subscription-level coupons
| Coupon type | What it targets | When it runs |
|---|
| Line-item coupon | One specific charge within the invoice | Step 2, before any subscription-level math |
| Subscription-level coupon | The invoice subtotal | Step 3, after all line-item discounts |
Multiple tax rates
When multiple tax associations apply to the same invoice, each rate is applied to the same taxable amount independently. They do not compound.
Taxable amount: $100.00
State tax (6%): + $6.00
Federal tax (2%): + $2.00
Total tax: $8.00 (not $8.12 from compounding)
Zero-floor rule
If coupons collectively exceed the subtotal, the taxable amount is clamped to zero. Excess discount is not carried forward to the next invoice period.
Subtotal: $50.00
Coupon A: -$30.00
Coupon B: -$30.00
-------
Taxable: $0.00 (clamped, not -$10.00)
Tax: $0.00
See also
