I've also had issues with BTC but only with this source. I buy the amount of BTC I need only when I am ready to send. So if my order is $800, I first do the conversion to determine how much BTC is needed to equal $800 and then do a screenshot. I send both funds and a screenshot to Opti.
On one order, Opti came back to me and said I was $50 short and sent me a screenshot of the conversion he took. The problem was that his screenshot was taken two days later when BTC dropped. That's not my fault. That's the nature of BTC, and that's what he chooses to use. The value of BTC goes up and down by the second. I thought this was very unfair because, say, BTC increased by $50 when he decided to cash it out. Will he refund me that $50? In the past, BTC was doing well and was steadily going up in value, so by the time he would get my BTC transfer, it would always be worth more than when I originally sent it. He never sent that increase back to me, but I'm supposed to compensate him when BTC drops at the time he decides to cash it out?
Now when I order I ask him how much BTC is necessary to make the purchase and send a screenshot of the conversion. I leave the conversion at the specific time up to him so that there is no question.