What do you do with a parent that doesn't pay required costs?

I am the treasurer for a multi-level troop. Last year, we had a parent who has an ongoing illness that has flare-ups, so she struggles to get her kid places on time/with the correct supplies, make meetings sometimes, sell cookies, etc. Obviously, we give her a lot of lee-way where we can.

However, last year, she would often "forget" to pay us back for things she needed to reimburse us for (tickets for events mostly), or say that she "didn't have it now but would get it later." Often she kept putting us off, so we would forgive it as it was small amounts each time ($25-$50). This amounted to about $200 or so over the course of the year. The troop just absorbed this mostly, or I paid out of pocket to cover it. I know, that's probably where I messed up, but we had already bought the tickets in all cases and they weren't refundable.

Late in the season, almost when the deadline for sales was looming, she decided late in the game that she was going to try to sell cookies. They had lost their original order form, and we had already made our cookie order, but we got them a new form and said we would go to cookie cupboards for her orders. She sold a bunch to a neighbor, but then said that the neighbor wouldn't answer their door when she had the cookies, and she didn't have the money to pay them back herself. We just took the cookies back and absorbed the cost (about $42).

How would you handle this in the new year? I don't want her daughter to miss out on girl scout activities or cookies sales, but we need to be better about this. It's not fair to our other members. I think we aren't going to be pre-funding the activities anymore, we are going to have the parents sign up and pay individually. I think we also need to get clear with her the week cookie sales starts on whether she is selling or not, and tell her we can't be starting so close to the deadline. What would you do? How would you handle it?