Is there a limit to how much I can earn with a single coupon?

Yes, there a a $25 per store per month limit.  This means that you can earn a maximum of $25 per store, regardless of how many offers you submit.  Further, coupons are eligible to earn commissions for maximum 60 days after becoming active.  After this period, the coupon is no longer eligible to earn commissions, even if it is still active. So depending on when the coupon becomes active, the theoretical limit you can earn with a single coupon is $75 ($25 per month, for 3 months).