I always use Eyeland in Daskalogianni street. If you walk along there from the market end they're on the left about 50m before you get to 1821 Square. You can't miss the shop: floor (almost) to ceiling window usually with prints on the ledge inside. Speak to Alex who does all the printing. Price depends on paper chosen - I always go along with their default - which, last July, was charged at about €45 per m² + VAT. They have a minimum charge of about €5 so that will be the price for A4 size and A3 about €7 inc VAT. (Those were the prices last July which may have increased slightly, of course). Tell Alex Philip from Kalyves* suggested them and you may get a small discount.
I belong to a photo club here and we have an annual exhibition and last year several members went to Eyeland for printing and framing and all the framed photos (about 20) I have on my walls were done there. The most recent batch, all roughly A3 size, printed, mounted and framed (in a good quality wooden frame) cost me an average of about €35 each - with a small discount.
Take your USB stick and Alex will copy the photo to their system, put it up on screen (it will print as seen there) ask if that's how you want it and tell you when it will be ready.
* Or Apokoronas Photo Group