I find it hard to imagine a reason a shop selling 100 tee shirts in a day would need any custom functionality since this is basically the exact use case all these OOB e-commerce tools are built for.
I've worked for such a place. Off-the-shelf solutions are trying to be everything to everyone and you can end up customizing the crap out of them to the point where it can become more onerous than just building your own. The place I worked was also print-on-demand service and had hundreds of thousands of SKUs as well as allowed customers to make custom products and we also hosted some peoples' shops. Shoehorning that into a custom solution was painful.
I work at a very similar place now that uses Shopify. Managing that many SKUs on Shopify is crazy painful.
The thing is is that custom ecom solutions really aren't that hard. The off-the-shelf ones are complex because, as stated above, they are trying to be everything to everybody.
In our B2B space there are tons of custom business rules. For example, you place orders per manufacturer and each manufacturer has its own minimum and reorder amounts and reqs on an order, promotions, business rules, etc. This is for business that have been around 25-30 years.
We did an evaluation on several out of the box ecommerce solutions and none of them were able to meet the requirements that absolutely had to be there. Shopify flat out said no, they can't help us, etc.
One of the first commercial projects I worked on nearly 20 years ago was a tshirt shop. And the precious company I worked for was a comparatively huge logistics startup.
I’m fairly confident I could spend a month or so writing a custom solution for a shop selling and shipping a few hundred tshirts a day that would save them enough money to break even on the software in a few years (compared to off the shelf solutions).
If I was starting my own tshirt company, I’d definitely do it.
