Agreed. Let her know now, and don't stress too much that she's trying to make you feel guilty. I think you can apologize about having to go back on the plan to help with the gifts, but don't feel that badly about it since the promise was made a while ago and without specific details of what the actual plan was.
DH and I are both honored that you'd want us in the wedding, but it's simply not possible. I'll be one month post-partum with full plate of kids and recuperating, and a still empty wallet. TBH, with the date now set on Saturday, 8/16 it's likely DH won't even be able to attend because he won't have any available days off at work at that time, and we can't afford a day off without pay. I'm sorry it's working out this way, and I feel terrible going back on our plan that DH and I would help transport the gifts, etc. but I wanted to let you know now that we can't commit to that after all, so you can find someone else who can help out instead.