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 ?
XMR.to has slightly lower exchange rates (usally around 1%) than the big exchanges, but even for small payments they cover all Bitcoin onchain transaction fees and their transactions usually will be included in the next Bitcoin block so they have a pretty good fee estimator and/or are well connected with Bitcoin miners.
When creating a new payment on xmr.to you will get a "secret key" in form "xmrto-xxxxxx" which you should write down and you will be able to track your order here: https://xmr.to/track
Orders up to 0.1 BTC will be processed immediately and your Bitcoin transaction will land in the Mempool usually within seconds, you will receive a transaction ID which you will be able to track in every Bitcoin block explorer.
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.