There's no set way of pricing shop work at a set rate, I only do commercial and do a lot of high stret work, things like frequency alter the price a lot, standard spec can vary a lot. If there are external tiles to the front of the shop do you include this in your pricing, I do. Try bump the price up and include signage, customers like this. There is a lot to factor in.
If a job takes 5 mins, it's easy money at £6, if a job takes 15 - 20 mins and priced at £15, this is still good, if a job takes an hour and pays £30 we still happy.
When doing commercial always try include anything extra to help increase price,
If a job takes 40 mins and is priced at £20, tell the customer to have it done twice a week for £30 or 3 for £40.
It's completely different from domestic.