VAT question

I am wanting to charge £4.99 for a shipping method however as one of my product is zero rated vat and the others are standard vat when zero vat product selected the charge is £4.99 which is great however when it is a product which is standard rated vat the charge become 5.99 as it has added 20% vat. I have tried everything to solve this but cannot think of anything. I either need a way to add the price inclusive of vat or a way to exclude variation ids?