currency changes when changing category

Hello, I hope that someone can advise on this issue, I have a website the link you can see above, the owner wanted to display the product prices in US$, the reason for using the plugin. However, I recently noticed that if you change to a different category the prices change from Dollars to Euros. It doesn’t do it on all categories.
has anyone had this problem and if so how did you fix it?

Ian

The page I need help with: [log in to see the link]