Should selling price depend on product quality or on work to produce the product if both not in positive correlation?
No one is &quot;right&quot; here, there is rarely a &quot;should&quot; in economics, it is not about morals.
When making a purchase, you are usually willing to pay an amount $x$ larger than the price $p$ which is larger than the cost $c$ it took to make the product/provide the service. You are arguing that price should only depend on cost. The store is arguing that if you are willing to pay more, they can charge more. Unless they have competition who undercuts them, they are right.
In the theoretical model of perfect competition, where there is a single type of product and firms face very strong competition the price would equal the average cost in the long run. I would argue the model does not apply here, competition is not perfect (if I am wrong, go to the competition). In case of imperfect competition it is normal to charge people whose demand is less elastic w.r.t. price more.

What you can do: pay them or find a better offer. If you are convinced that you are right, found a rival printing firm, undercut them and put them out of business.