I’ve always had my letter done via my MS nurse and never been charged for it. Keep the meds with you - never put them in the hold. I was really worried about travelling with them and didn’t do so for about three years but then took the plunge and wondered what I was worried about! Sometimes they ask you to open up the bag (particularly because they want to look at the gel/ice packs) but I’ve never had any problems. Just make sure you have all the documentation you need - I carry one of the patient information leaflets with the meds too, and obviously always keep the injections unopened in their original packaging.
Also, don’t let anyone try to tell you that your medicine bag is part of the inclusive hand baggage allowance. A medicine bag should be allowed as an “extra” piece as it needs to be carried separately and insulated. It means sometimes you can get dirty looks from other passengers when they realise you are taking up more space in the overhead lockers because you have an extra bag (my Copaxone carrier is the size of a small rucksack) but I’ve just learnt to ignore them and savour the fact that I have somewhere that I can also keep my drink and sandwiches cool. Lol