Does XMR.to have any extra fees when sending money ? How do I prove to seller that I send bitcoin to him ? After I pay to XMR.to with Monero and set the bitcoin address they need to send money to they will give me a bitcoin address that they used to send money to the seller ?
You can just check the target bitcoin address on a bitcoin blockchain explorer like https://chain.so/.
There you can see all incoming transactions that are send to the address you put in to the xmr.to site.
When you send monero to xmr.to it will exchange it, there are no extra fees from xmr.to They will exchange your monero acording to thier indicative rate that is displayed on the site.