Can I use an agency to extend my ED Visa instead of going to Chaeng Watthana myself?

Have an ED Visa which needs to be extended this week. I want to avoid another full day of Chaeng Watthana hell. My school tells me it's not possible to send a courier to extend on my behalf and besides that they will only ever grant 60-day extensions (as was my previous extension).

I've read other sources however saying I should be able to pay an agency to apply for the extension on my behalf and that normally an extension would be granted for 90 days.

Can someone tell me what's true? And if it's possible to send an agent to apply for the extension, could you recommend me a specific agency and tell me how much I should be expected to pay?

Last time the whole ordeal took me around 13 hours, transportation included, and I'd do just about anything to not have to deal with it again. Thankful for any advice.