Postcardshop with standard 5 cards to choose

-- url of the page with the problem -- : debestenatuurfotograafterwereld.nl/new/i...product/36-postcards
-- HikaShop version -- : 3.2.2.
-- Joomla version -- : 3.8.4
-- PHP version -- : 7
-- Error-message(debug-mod must be tuned on) -- : Postcards shop problem

Hello
I have a concept postcard shop for a client of my. The first 5 postcards are € 14,95 incl TAX and shippingcosts. I have found the option possibility so the visiter can select/order the 5 postcards out of 20 cards.

But what to do after the visiter wants 1, 2 or 4 or 8 cards more. The price of the next cards is € 2,50.
I have put the Quantity per order on 5. So when they choose for another card the price of that card must be € 2,50. Maybe after 10 cards the price of the order will be € 2x € 14,95 = €29,90 ? How can I fix this you think?

Mus I also do something about the shipping costs? Because that will be more if the order is getting bigger.

A difficult problem?