Name-changed. I’m purposely keeping some details vague as this could be quite outing.
Please don’t flame me 
It’s not a massive issue, but I feel really bad and would like some advice on how I can resolve this.
I recently started tutoring a new client.
Stupidly, I didn’t check my current prices for how much I charge for different levels. This is because clients usually ask me to teach one specific type of qualification. When the client contacted me, they said it was to tutor them for that specific qualification so, stupidly, I didn’t think to check my other prices for other qualifications(!!) I completely understand it’s my fault.
I told the client that I would double-check my prices on my phone. I only got a chance to try doing this when the tuition had ended. Unfortunately, by this time, my phone had died. Again, I totally accept that this is my fault.
I ended up charging the client slightly too much money as I mistakenly asked them to pay for the price I charge for the other qualification.
How should I resolve this?
I need to give the client some of their money back. How can I explain this?
Can I tell them I mixed up the prices for the two qualifications? I feel like this would make me sound really unprepared and unprofessional, and I feel the client wouldn’t take me seriously after this. But I can’t see a way around it.