If you create a coupon for an amount larger than the monthly cost of a subscription, the total amount due will be a negative number. For example, if a user tries to use a $10 coupon for a subscription that costs $5 per month, the amount due will be (-$5). The paypal_r plugin will pass the negative number to PayPal, and the user will get this error message from PayPal: The link you have used to enter the PayPal system is invalid. Please review the link and try again. The PayPal URL includes this: ... &a3=-5.00& ... (URL abbreviated for clarity) It's minor, and is usually easy to work around, but it should also be very easy to fix. I can see an instance where someone might want to issue a coupon that's valid for up to $10.