Adding price with Shopify Coupons

[Chbadr]

Hello, i have a store with prices tax excluded, usually my customers don't ask for the invoice so i don't have to add or pay the VAT, but sometimes the customer ask for an invoice then i have to add 20% of the total price, however i can't apply the taxes on all my store because not all my customers ask for it.

Is there any way to create a negative coupon like to add 20% of the price when applying it ? or an app that makes a 20% value added at checkout optionally ? 

Thanks

[Chbadr]

Please need help