Popcorn has great mail order possibilities but you might need more room than you have to get a decent amount. So do dried beans but a 1/4 acre would at best yield you 20 pounds of beans at most and that would not make you much money.

Garlic might work well for you. it can be mailed easily and if you grew a 1/4 acre of it you'd have thousands of bulbs. but garlic really needs to be planted in the fall, though you can plant it now for a fall harvest (if you can find enough garlic to plant).

If you are a seed saver, seeds can be a good seller (though you won't get rich)

Local harvest is a great site for marketing your crops. They give you space to make a web page (and it is way E-Z). And it is free

Here is a link that might be useful: Local harvest

Garlic & shallots , We have about 50,000 on less than an acre. They both have long shelf life and can be mailed. You can also make garlic & shallot braids and garlic powder. It is hard work: hand planted, Hand harvested,cleaned and WEEDED! You won't make money on it at first because the seed is intially expensive But after your first crop you can save your own seed. We went 3 years before we sold much garlic just saved seed and sold only the smallest.
If you are interested come over to the Allium Forum For lots of advice and ideas
The Garlic Lady

There is unlimited things you could offer for sale. Keep in mind the marketplace swings so what you start out growing may not be what you end up selling - or - the way you see things today may not be as they end up. I would go with herbs myself just because you get more from less work and they are popular.

The only advice I would like to pass on is to beware of mail order pitfalls. There is nothing wrong with shipping plants by mail as long as you obey the law but the price of shipping is high and constantly changing. It is impossible to claim on your website what shipping charges will be unless you also OWN the shipping company. All the major carriers will offer you a contract spelling out when they will pick up from you and how much shipping your goods will cost. You will then do the math and arrive at a set price to charge your customers. The cost of gas will rise or the weather will hit a mean streak and the contract will be voided by the carrier leaving you in a lurch, unable to raise your posted shipping fees. You now have customers ordering from your website expecting the posted fees (which they think are too high) and you have to take a loss just to get the product out the door. It happens even to the large companies. You get to sign THEIR contract, they won't be signing YOURS. Myself, I stand in line at the post office, postal rates are cheaper than everybody else but they won't come to your door to pick up and they raise them all at one time - no surprises.

If you are short on space, you might want to consider selling flowers and perennial plants using the winter sow method. G/W had a forum with all the info you need to get started sowing seeds OUTDOORS. You don't need grow lights, you need milk jugs! It's very addicting, it's very inexpensive, and it will give you more plants than you know what to do with!

Here is a link that might be useful: Winter Sow Forum

"Myself, I stand in line at the post office, postal rates are cheaper than everybody else but they won't come to your door to pick up and they raise them all at one time - no surprises."

Just a correction there, the post office WILL come to your door to pick up...go to usps.com and get more info if you'd like. Just request a pickup online and print your labels.