Swimming Binders and Transphobic Mother?

I've been looking around for a swimming binder to wear during the summer while swimming and I came across the Underworks Sleeveless Swim Top. If my calculations are correct and my mother didn't spend my money I should have enough. I own a regular binder but the only way I got it was through negotiating and lying. I said that I needed it for cosplaying with my friends, but I don't have an excuse to use this time. Saying I'm trans and I have top dysphoria wouldn't work because my mom doesn't accept me as a trans male and doesn't understand the importance of passing as male. She tells me to ignore my dysphoria and to accept the fact that I'm a girl. How should I ask her? Can you guys help me with an excuse as to why I want/need a swimming binder? Also if you've used this particular swimming binder, can you review it? :help: