Go ahead, walk all over me

So, I'm a pretty nice and helpful person I think. But lately, I'm beginning to think that, while I see myself as nice and helpful, others see it as a way to get someone to do their work, guilt-free.

Lemme explain.....

I'm in a wedding May 1st, and there are 2 other bridesmaids in the wedding too, and they both live out of town. So, I am pretty much the one doing most of the work with the showers and stuff like that. The bride has a sister-in-law, who is very involved with the wedding, too. Maybe too involved, but that's really a matter of opinion. Anyways, she got this wild idea to throw the bride's shower....fine by me, since that took the responsibility off of me (I have a feeling she wouldn't have let me had i offered, anyways). So, she finally booked the community center where we are haveing the shower, a few weeks ago. And so I ask her what day she booked it for? April 11th....at the time, that gave us slightly less than 3 weeks to plan this little bash. She said she had the guest list all planned out. I told her, if she needed my help, to just call me.
Well, about a week later, I called her and asked her how the invitations were coming along, and she said she hadn't even started them,a dn it would be anothe week before she did. I thouhg that it was ridiculous to give people a week's notice for a shower, so I offered to buy and write the invitations, if she'd pay me back. She thought that was a good idea. So, she gives me the guest list, which has NO LESS than 146 addresses on it. ABSOLUTE MADNESS!!!!!!!! But, I figure, whatever...not my shower. SO I go buy the invitations, which she and the mother of the bride told me not to address, since they would just hand them out. I did all this, and got the invitations back to the mother of the bride earlier this week. I figured I was done.....WRONG!

The mother of the bride called me last night, and said there were about 10 invitations they needed to mail, and since I still had the address list, would I mind picking them up at another sister's house (who was helping with the wedding), and putting the addresses on, and mailing them. I figured, why not? Ten invitations is no problem, takes no time, and I have 10 stamps.
So, I go pick the invitations up today, and the bag feels rather heavy. I get in my car, and count them. These things must multiply like rabbits, because there were not 10, there were 45!!!! How in the world can you confuse the 2 numbers!!
So long story short (but it really wasn't, was it?), I have to call the mother of the bride and tell her that I will gladly put the addresses on them, but buying more stuff is just asking too much. The bride has owed me over $50 for over a month for stuff she asked me to get for her reception....we're not millionares, you know?

Anyways, what's the nicest way to say, "Go get your own stinkin' stamps!"?

And how the heck can I stop letting people feel like they can walk all over me, or ask me to do anything, and I'll do it?