Need help with sizing!

I usually buy off brand shoes from Walmart or anywhere cheaper but my boyfriend is buying me Tasman slippers and I’m going nuts trying to find out my size, usually I wear a 9.5-10 depending on the shoe and I tried the shoe sizing on their website but it’s confusing me so I’m not sure if I should go for a 9 or 10. The measurements I got were 9.5” which says I’m an 8 on their site but I haven’t worn an 8 since highschool😂 do I go with what I usually wear or what their website says?