Hey there!
I think you should look again, of course coinb.in allows custom fees, it just appears you don't understand how fees work.
As you stated, what is not sent is used as a fee, this is how the Bitcoin protocol works, what is not spent is used as a transaction fee - it's simple. To avoid this, you create a second "change" address where you spend the change back to your own address, thus lowering fees.
For more information on fees and "change addresses" please see
https://en.bitcoin.it/wiki/Change and
https://en.bitcoin.it/wiki/Transaction_feesPlease remember coinb.in is meant for advanced users who understand the bitcoin protocol and whilst I don't want to discourage you or anyone else from experimenting with this project and increasing your understanding, you should be extra careful as you clearly aren't fimilar with the protocol as you have shown by your lack of understanding and complaining.
All the best.
You are right and electrum wallet automatically add my address to the tx as change address and in coinb.in we need to do it manually now i understand.It would be great option to have this in coinbin wallet too so we don't have to manually enter the change address our own sending address should be set by default to change address but i understand you if it's meant for advanced users.