Change Prestashop Tax Rules

We are an online auction website and have some specific requirements which Prestashop does not support out of the box. We auction products, and we charge 10% commission on top of the sales price for our customers. We currently use the Tax feature to do this. We set Tax to 10% and renamed the Tax to 'Buyers Commission'. Now we need to pay UK VAT (20% tax) on the Commission charge and we need to pass this on to our customers. The 20% tax should be applied to the Buyers Commission (10% of product price), and also to the shipping costs. We also have an insurance module which tax should also be applied to. Here is what the invoice should look like: Product A: £100.00 Product B: £50.00 Buyers Commission (10% of products): £15.00 Shipping & Handling: £12.00 Insurance: (3% of products): £4.50 Subtotal: £181.50 UK VAT ( =(Buyers Commission + Shipping + Insurance)*0.20 )= £6.30 Total: £188.00 This should be applied to all invoices in Prestashop. One idea is to change the Prestashop Vouchers to a % of the products and use as the buyers commission. Then apply the Prestashop tax feature correctly. Please read these requirements carefully. Do not bid on this project unless you are 100% confident you can carry out the work. The project must be completed on time.