All you’re doing is putting it in the post, otherwise it’s a normal sale.
Compared with a shop sale, you have wrapping, carrier's charge and maybe taking it to the carrier's office, depending on which service you use. Premises, finance and labour costs should be pretty much identical if you're a shop which does mail order rather than a mail order only outfit. Do you really think you can wrap and post something for less than £6 over the cost of passing it over the counter?
Compared with a shop sale, you have wrapping, carrier's charge and maybe taking it to the carrier's office, depending on which service you use. Premises, finance and labour costs should be pretty much identical if you're a shop which does mail order rather than a mail order only outfit. Do you really think you can wrap and post something for less than £6 over the cost of passing it over the counter?