A bottle is £6.76 with free postage. Say 1/3 of that for one sleeping bag. The cheapest postage possible by using on-line rather than post office, will cost £2.90. What quantity of materials are you offering to cover the £5.15 it will cost them?
I think this could be another case of false economy.
Why not buy a bottle, then offer the remainder here, or save it for sale at the next Moot....or just hang on to it for the next time? It does not go off with age.