What expressions should I use to reply "sorry to be a pain, but …" courteously
Solution 1:
All in a day's work. Don't mention it.
Should convey what you want to say pretty courteously.
Solution 2:
"It's no trouble"
"Don't apologize!"
"Don't worry about it"
"That's fine, it's what I'm here for"
"No problem"
"Sounds great"
"I'll get right on it"