Tough Decision, i need help brides

Sooooo, I have a bridesmaid that is my cousin and has been so helpful to planning my wedding. She even helped find the shoes and dresses that was all I asked for them to do on their own. Fyi it's a destination wedding for '24. She went as far as finding the place to get the dress from.
Now.... it's time to start looking to buy dresses and she is adamant about buying her dress from Amazon. I am sooo upset. Not because it's from Amazon but because that was never the plan and I'm not changing plans to appease her when this was her plan from the beginning. We all agreed on 1 dress, 1 place and the price. Haven't heard from her in 2 months. Giving her till December 31st before I "kick her out". What should I do???