When I bid..

[wrawrawra]

This is going to be stupid, but the "YOUR BID" box is the max number I'm willing to bid right?

So, let's say there's an item I want for $180.

I input 200.

The auction ends.

I pay 180 + 2.50 or am I paying 200 flat?

[Cupid]

You have the right idea... Ebay only ever makes you pay, at most, one bid increment more than the next highest bidder.

I would also like to recommend that you read the FAQ section (accessed via the FAQ tab above) as this and many other common questions are answered there.
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Mark