Bitcoin rawtransaction fee Create a raw transaction createrawtransaction (transactions, vouts, amount and where to send) 
then specify the commission for the miners fundrawtransaction feeRate But when I start checking or signing and sending, the commission becomes bigger .... What is it connected with? 
p.s sorry for my english :)
The transaction fee calculated for the transaction by fundrawtransaction cannot actually predict the transaction's final size. This is because signatures are not fixed in length and can be smaller than what is predicted. However, in order to be sure enough fee is paid, fundrawtransaction will always assume the maximum signature size. This means that sometimes the signature will end up being smaller which means that you end up overpaying a little bit in fees. The feerate will be higher because the fee has stayed the same but the size of the transaction is smaller than predicted.