Caveats

Possible inaccurate coupon total

FastSpring doesn't have support for passing WooCommerce coupons. Consequently the workaround is to divide the discount among all the products. In most cases this works fine but in some situations this can result in a small discrepancy (such as $0.1) in the final total.

For example, if the customer purchases 3 items at $20 each for a total of $60 and a coupon is applied for $10 off then the FastSpring total will be $50.01 ($10/3 = $3.33 applied to each item = $16.67 each x 3 = $50.01).

In general most discounts will not cause this error and if it does happen it's unlikely that your customer will balk but if this is an issue then the solution is to use FastSpring coupons instead.