When auctions are running, you can easily apply a maximum bid to a running auction.
To do this, select "Custom" on the bidding window at the bottom of the stream you're viewing.
On the custom bid screen, you can select to submit a regular bid, or to set a max bid for this specific item. Submitting a max bid will result in automatic bidding up to that bid total, based on Loupe's auction billing increments. If another user has already bid higher than your submitted maximum bid, you will immediately receive an "Outbid" notice.
If your maximum bid, or a value within your maximum bid window, win the auction you will automatically be charged for that auction item. Once you've placed your bid, the maximum bid amount will be shown at the bottom of the auction window.