Small retail shipping sucks at times.
The successful larger (cheaper) players tend to be located close to the shipping firms locations, so have zero collection fees, and likely preferential rates. You can't compete on price.
Wholesale doesn't start at $51.00; wholesale is what your larger commercial customers pay.
I think you need to compete in your niche - quality, premium work - the thing that sets you apart. Having a minimum order of $51.00, and an extended shipping time could be made a feature, not a bug of your service. We ship within seven working days, allows you to gang up the shipping orders, and have UPS collect. If the customer "needs" it in a couple of days, have cost related premium shipping.
Is there something that you can do to uplift the $51.00 customers to £75.00, that wouldn't kill off the small orders? Looking at the combination of print sizes (multiple A3 works great on 24" rolls, but might be a hard sell in freedom units), to media, and making a sharp price break for the uneconomical sizes for media, packaging or shipping?
We recently had a print sale, and made the mistake of offering global shipping. We went for 10x12" in tubes for single prints that fitted into small parcel global, with multiple prints flat packed between pairs of corricor (ordered from our framing materials supplier, cut down ont the SteelTrak) in polyprop bags.
The prep for tariffs, etc was truly awful, though ultimately profitable, though what is left of my hair is now more white/grey than before.
Keep going, refine, and remember that people buying quality, even artisans and photographers, often have disposable income that allows them to purchase premium products, like yours.
Cherchez Le Creneau.