My grandmother is making my bat shat crazy over wedding details!!! HELP !?

We are 3 weeks off of our wedding date. At the moment I am finalizing my florist (down to 2) I know it's close before I procrastinated a lot on that part. My venue provides these hideous cheap centerpieces with fake flower toppers. Well actually the center piece stand itself is gorgeous; the flwoers on top howover are disguting. But for monies sake I was acutally going to cut back in that area and use the ugly centerpieces toppers rather then buying real flowers. My grandmother very generously insisted that I pick centerpieces that I liked and that she would pay for them. Well now we are down to the final 2 florists as I've mentioned and she is making me NUTS! I am paying for all of the bouquets, corsages, ect. She is paying for just the centerpieces. The two places we are looking at have a $10 difference in price for the centerpieces and there are only 13 of them. So a difference after taxes of about $150. The total bills however are different by about $600. The more expensive centerpieces are from the more expensive florist. Both of the florists are extrememly experienced. One previously worked for a prestigious hotel in our are, the other has worked in then purchased the shop she runs for about 15 years now. I feel that since the majority of cost difference is affecting what I am paying for I should make the decision. My granmother had worked for the company of the more expensive florist before the now owner was even around, when the original owner had it. This is the florist she wants me to choose. Basically her only reasoning is that when the woman called her she was very specifc as to what quantiity of what specific flower we are getting in each item for the price. Which is a very nice detial however the cheaper florist who I prefer once asked for more detailed info gladely gave it to us. And the number of flowers for the price difference is almost nothing. Yet for some reason even though the cost almost solely affects me; she thinks that she gets to make the decision. And if she did go with my choice; I'd be saving her $150. I love my grandmother and she has been very generous to pay for a very large portion of our wedding (something she wanted to do for me for ym whole life); but in these last few weeks she is trying to impose all these changes on things we've already either purchased or decided upon. I can't take it any more. Why would she be like this? She's been included for much of the planning so she is certianly not left out. It's just that she offered to pay a certain amount and I told her thank you and that I wouldn't go over budget but she had to promise to just let me get/do things how I wanted them. And now that much of what we've done is not to her liking she is being a tremendous pain in my Azz!