I’m a freelance photographer. A friend’s band asked me to take pictures for their album. I offered to do it for free but the whole band insisted that they pay me and to send my price, which I did. It was a cash-in-hand, friends-rate figure but still went into a few hundred or so dollars.
The guitarist wanted everything to be ‘official’ so requested me to invoice the band properly, I explained that doing so would raise the amount by 20% for tax. He was totally happy with that and understood completely and thought my price was still really a bargain for them, which it was.
No surprises what happened. The guitarist is super-pissed when he eventually sees my invoice as it’s 20% higher than what he expected and manages to persuade the rest of the band I have somehow ripped them off, so he grudgingly pays me and they have never spoken to me since.
Never work for friends.