standard transit arrangements

Hello I just wanted to find out what the "standard" arrangements are when one buys transit traffic. I've been left in charge of this now, without haven't been doing this before :-( So what normally you buy x amount of bandwidth over a physical line e.g. 45Mb/s over FastEthernet or so. They then charge you for excessive usage based on 95th percentile. Is there anything else I need to keep an eye on? Thanks Sven