When using the PP Standard payment extension, the cart posts products and shipping as separate lines to PayPal. Sometimes a mismatch of the cart total occurs when PayPal rounds-up (or down) each item's tax amount separately and then adds them together instead of calculating the tax per order (as IMO it really should do and as every other accountant/module/business/cart/gateway/sensible person does - but heh, we are talking about PayPal here!)
This modification posts the cart total to PayPal instead of a multiple of items so that the total always matches correctly even though it shouldn't need to :(