You should pay VAT on light aircraft parts. Not duty. Royal Mail always get it wrong and if you query they hit you with 6 weeks plus delay to 'send back to HMRC'.
The £13.50 charge is more expansive then Fedex. Shipping with Fedex is more expensive however you can track and they ship out to you straight away. Rather then send you a letter about charges.
Saying that Fedex are not always 100% correct.
End-use number helps but not always. The last time I used Royal Mail they charged duty saying I had not put end-use number on parcel. When I collected it the number was in large font on every face of the parcel!
DON'T USE ROYAL MAIL or PARCEL FORCE for imports. Domestic they don't know what they are doing international imports.
Within the last few weeks HMRC have change the rules on duty free aircraft parts import. You can do up to three a year. More and and you need end-use number - which they expect you to have VAT account etc also...