FBA vs FBM conversion factor for items below £$20 mark




I am selling supplements via FBA at the moment, however in my opinion I am loosing a lot of sells because of £$20 mark, after which you will get free delivery. There are many customers would buy it as long it would be free. Does anyone knows what kind of conversion difference is in this case between FBA and FBM. Will it be any difference you think?
Also what about EFN; is there any better/cheaper services for European fulfilment from UK than Amazon FBA?
Appreciate any comments.