cart voucher issues and wrong info sent to paypal..

Hi I have a few concerns with coding a voucher system:

lets say a customer, has a voucher for £20 and the product they are ordering costs £10 - so if they use this voucher on their order then the grand total for the order will be -£10 - So how will that reflect on purchasing through paypal? How can a calcuation be set up in ecart that will deduct the voucher amount BUT not below 0 (so that there are no negative values in the grand total that gets sent to paypal?

....also another issue I had which I cannot remember how it happened...but I think it had something to do with a customer getting free delivery...if they select for free delivery when the order gets posted to paypal, I noticed that paypal creates a made up delivery charge of £2.99 and once it made a delivery charge of £9.99 which is odd as i havent set up any delivery charges for both thoses values (I only set up £7 and 99pence). So how would paypal get £2.99 and £9.99!? very confusing.

lastly, when a voucher has been used...when it gets sent to paypal, the correct grand total is displayed on the paypal page..however the product costs are halved. so its odd that paypal seems to half the product costs. the main point is that the customer is paying the correct "grand total" amount in paypal. it just may confuse them that the products on paypal screen will show as half the cost.

Chris