checkout url not constant

I have setup a main menu in joomla for the checkout. this menu item is under a menu item called store.

so www.domain.com/store/checkout

only problem is if someone is browsing in a category and then adds a product from that category, the popup comes up and they say go to checkout.

the checkout address is not www.domain.com/store/checkout but instead it is www.domain.com/category/sub-category/checkout

same thing happens when i click the 'Go to Checkout' button in the cart module.

how do i make the checkout page have a constant url??

woohoo!

thanks nicolas!