K.P.
This is in poor taste on their part. The rule of thumb has always been that you don't invite anyone to the shower who is not on the guest list. That was a shameless "gift grab" on the bride's part and her mother should have either:
1. Reviewed the guest list and eliminated the "shower only" invites
2. Spoken with you PRIOR to the shower to explain that they were limited on the number of guests due to the venue (not financial- that makes it worse), but that she hoped you would join them at the shower to celebrate and no need to bring a gift. You would have brought one anyway, but at least you would have had a heads up!
The gracious thing to do would be to send a card (no gift) congratulating them on their wedding, but that's really up to you.