Alternator problems. Best option?

[7aris]

The alternator on my 07 Yaris is charging intermittently. I don't mind replacing the alternator myself, but I'm worried if it's going to fix the problem

6 weeks ago, I had the oil changed at the shop, after leaving I noticed the battery light was flickering. Took it back to the mechanic, he checked the voltage and it's at 12.24v. Mechanic said it's was the alternator. While the mechanic was poking around, I noticed the terminal on the alternator looked corroded. I glanced at my dash and noticed the BATT light was off now and mentioned it to the mechanic. He checked again and it's now charging at 13.6v. He said he didn't do anything so I took it home

After I got home it began flickering again. I clean the terminals on both the battery and alternator, reconnected everything and the car is now charging at 13.36v.

Flashforward to Last Sunday, I went to go start the car and the battery was completely dead (8v). Replaced the battery (old one was 9yrs old), and checked that it was charging, 13.6v

I decided to keep an eye on the voltage. On Tuesday (7/22) after Parking the car at home, the battery was at 11.73v, I started the car and the alternator was charging at 12.01v. I jumped charged the battery and got the voltage back to 12.7v. Brought the car to the mechanic. The next day he calls me and says there's nothing wrong after load testing. Mechanic says he can still change the alternator for $500 (remanufactured). I asked him why it wasn't charging before and he said it's some type of intermittent problem. I thought $500 was too expensive so I took it home. Checked again, now it's charging at 14.1 volts but after 10mins it drops to 13.44v

What do you guys think?