Fees set up for transactions set up

I have a form set up to add $1.38 to the price for each ticket bought, to cover online processing fees. But it really needs to add 1.38 for each transaction, so that someone can buy multiple tickets in one transaction and not get charged 1.38 for each ticket. How do I set that up or word it as a condition? Thank you.

I see this was answered previously, please disregard.