First, sry for my english, i'm french ;)
So what is the problem ?
When i activate wpsc-simple-shipping plugin on my wordpress website, to add options on shipping on wp-ecommerce plugin, i can easily put my fixed prices to ship my products. But after several times, there is an error. If you add products to the checkout place several times, and you want to pay, the 503 error is on the screen ! And after that, the only solution is to delete cookies...
What is the link with wpsc-simple-shipping ?
If you have the 503 error, you desactivate the wpsc-simple-shipping plugin and... everything works !
I let you answer.