There are several hundred companies that will guarantee you to get on a GSA Schedule and whatnot. I get calls from them weekly.
To be on a Schedule Contract you must provide fixed pricing on standard items, in other words nothing custom.
We do a lot of federal work for the DOD for all branches of the military but I only go after custom projects which we have to bid on. We also do a ton of smaller things that they do not have to bid, on that stuff they just call us. On the smaller stuff that just takes sales efforts to get to know people in different areas to let them know who your company is and what you can offer.
On bigger projects it can be an itimidating and daunting process the first time. There are a LOT of steps you must take and procedures to follow and whatnot to put your proposal together. You will also need to get setup in multiple systems like CCR, ORCA, WAWF, ELPS, FedReg, etc, etc... All a bunch of different websites with complex requirements to get your company setup in. They are streamlining some of this though with a new consolidated system which is being implemented right now called SAM. That will be nice as it will combine a few of the systems but it's still a bit complex and you have to update your company info every year.
What those companies that called you though, you can do every bit of what they want to charge you from $3000 to $10000 or more to do yourself with some time and effort to learn the ropes of federal contracts and contracting which you would ultimatley need to learn anyways. They are NOT government entities, they do NOT represent the government, they are private businesses out to make BIG profits. You do NOT have to pay a single cent to get a federal contract. You will however have financial costs in going for federal work though in time and effort to check the qualifications, attending any mandatory site visits, spending time putting your proposal together, and whatnot. I will say this much, once you have done it a couple of times and are setup for everything it's really not that bad and it can be VERY profitable and rewarding.