fix product rounding issue

We’ve encountered an issue in the payment system when dealing with live recurring products that include discounts.

The total amount paid across all invoices for a subscription doesn’t always match the expected total amount. This discrepancy is due to rounding errors—a small difference appears when summing up multiple invoices for the subscription.

Normally, when there’s no discount, we handle this by adjusting the last installment to account for the rounding difference. However, the issue arises when discounts are applied, as the adjustment logic doesn’t currently handle that case correctly.