Never worked with an agency before as a client. Ignoring the small potential of an LE issue, I see a lot benefits over the ongoing problems and concerns I have each time I want to try a new independent and their approach to screening. Also aware that often I don't really.start the communication with the "independent" provider but a background person. Rates for agency, let's just say BI, seem rather low for usually well reviewed providers. What are the expectations for additional payments once you meet actual provider. What kind of payment/tip is expected/good practise/proper behavior?

BI just charges the listed price, usually 400/hr. The only add-ons are things like Greek which is almost always an additional cost with any provider who offers it.

Agree with what AnarchistCatOwner said. BI offers women with typical GFE and from my experience BI is very reputable and I highly recommend.

Fully understand, but have read about tipping several times with BI recently, at least once with provider asking for it before session.
Fully get the idea to pay for extra.  
Also don't mind $100 on top of the $400 that seems to be the going at BI, just don't want to get blindsided with crazy upcharges.

I’ve seen about 5 BI girls, some multiple times, and never experienced that. Total guess but the review you saw asking for a tip at the beginning might have been for an upgrade and the reviewer was just being discreet about what it was for. I wouldn’t see that girl your first time using the agency just to avoid any confusion but I can tell you that with Eve/Aiza/Lily/Maggie that 400/hr means 400/hr full GFE.

That is if you don't want any extras. BBFS, anal or video with Aiza is sure to cost you more than $400. If you there not to do any of those things than sure: $400 for the full GFE is the price.