Looking at Royal Mail .. 0.5kg about 4ukp ... that would be 8 ukp for one there and one back.
Looks a bit pricy to me ... Australia Post do a 0.5kg satchel for $1.25 for a 5 pack.. so that is $0.61 each anywhere in the country (somewhat larger than the UK). They are a plastic bag with post paid .. should be suitable for damp things https://shop.auspost.com.au/pack-and-po ... e-satchels
Think your pricing is well out £2.90 for a 2kg parcel, that's paying over the counter. One has to weigh up if spending £xx is better than all the hassle of taking washing bits, actually washing and drying shorts every day and worrying about drying them, which was the question posed by the OP.
Personally I have enough shorts for the touring I do and take them all with me, I don't wash kit, it's too much hassle and means I have to take other stuff to wash it properly.