I use MarketPress and cart cookies don't clear properly when this plugin is active. At times I can remove items from the cart, it will say the cart is empty, and then when I return to the cart, the items are still there - and other times I can add items and the cart will remain empty. Deactivating SLP was the only thing that corrected this issue.
Does SLP have conflicts with any other e-commerce plugins? Is there a line of code I can add somewhere so that these 2 plugins play nicely together?