Shipping when offering diffent sizes

[APieceOfFelice]

I hope someone can help me with this.
I have all of my prints listed and offer different sizes of one print.
However I can not assign different shipping costs within this listing.
For instance an A4 print to USA will cost me $6,80 but an A3 print willl cost me $19,60 Just because of the size (not weight).
A big difference as you can see. Now, I don't want to set it to $19,60 for all because I don't want to make customers pay for something I'm not using.
But I don't want to set the overall cost to 6,80 either because when someone does buy the bigger print I have to pay the 12,80 myself.
I tried to see how other people do this but they all seem to have a fixed shipping price.
How do you decide how much to charge?
Do you set a fixed price? Or should I maybe do it so I refund the extra shipping costs if someone buys a smaller size?
I only have one bigger size in my shop. All the others can go for the lower shipping rate.
(Im based in the Netherlands by the way, don't know if that matters)
Hope you people know what i'm talking about and are able to help!

[FavoritePlacesPrints]

Not sure I can help since I'm located in USA and mainly ship in USA.

I ship all of my prints in both sizes I offer (8" x 10" and 11" x 14") in the same size shipping tube and ship USPS First Class so price is about the same anywhere in the US.

So maybe start with using the same size, type package for all of your prints so your pricing is about the same. This may not work for you since your price to ship is so much less with A4 size.

Here's the shipping tube I use to ship all of my prints:

https://www.uline.com/Product/Detail/S-1028/Tubes/White-Tubes-with-End-Caps-3-x-15-060-thick

I can roll up to three prints in a tube (after inserting in waterproof plastic sealed bag). If I ship more than 3 prints I ship them flat (sealed and with backing board) and then ship in a box. Shipping usually costs more than I charge the customer when I ship the prints boxed but I make enough money from the larger order to justify the small loss in shipping fees.

Hope this helps.

[elevencorners]

Could you create different listings, with different postage costs, for the two different sizes of print?