If you call agents and tell them about the offer, they will often jump at the chance to take you on. After all, it's a quick way for them to make a buck. Agents are paid by publishers. Usually, when they sell your book, the check is written out to the agent. The agent then takes his percentage of this amount and sends you the remainder.
Some agents also pass through charges for expenses to their authors.
This can happen in different ways. These include charges for such things as Express Mail, Special Couriers, and other items that are not the usual part of doing business. Regular postage and copying costs are not extraordinary. Agents generally deduct these expenses from any money due you. There are many sources, of varying degrees of reliability.