I always deal directly with the listing agent and make sure they know that I have no other realtor and they will not have to split the commission.

I wouldn't pay a large sum or upfront commission because your tenant could bomb out.

I've heard on the podcast as well that some people are agents and still use agents and give away the commission which is a great way to help build relationships.And tell your agent that he's too slow and if he doesn't adjust let him go unless there is something that he is giving you that you can't find elsewhere (which i doubt)

-I work for a flat rate broker and can discount my commission percentage to whatever I want.

I am about to close on a wholesale deal, and the buyers are asking me to give them an “Invoice for Commission” for the amount of my assignment fee.

You can inquire, but don't tick them off.If there is no satisfaction, there is always the RE Commission, but going there means kissing that property good bye I'm sure (as well as any future properties that agent may be involved with). :)   

Also, Commission Rule 535.4(e) makes it clear that all solicitation work must be conducted by licensees.

I have heard they vary widely from commission based to flat fee. 

I would to PMC to cancel the contract or your filing a grievance with the real estate commission.

And make sure you don't pick one that gets paid via commissions on the things you buy.