Invoice Calculation Order

The exact sequence flexprice uses to calculate invoice totals: line items, discounts, wallet credits, tax, and final amount.

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)

Two 10% coupons produce a 19% effective discount, not 20%.

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

A subscription-level coupon sees a subtotal that has already been reduced by any line-item coupons.

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

• Coupons
• Taxes
• Wallets