PayPal Shopping Cart Didn't Work Previously

I would like to install a simple PayPal shopping cart on one of my pages, however, the last time I did, my whole site locked up and I have had to rebuild all of the pages. I changed from Weaver II theme to Twenty Ten on the advice of very, very helpful Forum folks. . .they said it should offer few if any problems. Does anyone have any recommendations as to which shopping cart works well with Twenty Ten theme? Last time, I had everything looking good but the page would not redirect customers to a thank-you page once they completed their order. When I tried adjusting settings to fix that, my pages all defaulted to only one page, menu tabs quit responding, and I became unable to login on the WP Admin page. . .don’t want a repeat of that disaster, so would like to get some advice, please, before I try again on a shopping cart.