It's a very nice development for a person to design his own T-shirt, not a expensive price of 20 dollar, but it caught my attention; Why is not Bitcoin paying?
Dude there are many online logo designers site available in online. If you able to do by own you can only need to spare the money for the printing the t-shirt.
I do not think that it will cost more than 5 USD to print in any photo maker store.
However, if you open a e-store to buy the btc designed t-shirts means you will be able to make more money.
5 USD is a fair price to get t-shirts with its own design
in my country 5 usd is the market price,
because making shirts if the price 20usd spelled out very high compared to me ordering my own design in my country
so naturally the OP should correct for the price part