(FORTUNE Small Business) – My favorite part of a store is the impulse rack by the cash register. I have spent hours ogling the array of tabloids, gum, and candy lodged next to that last stop in the shop. I even stole my first pack of Bubble Yum from checkout lane No. 3 of the A&P (my impulsiveness was clearly overdeveloped, even at age 6). High-margin, low-cost products: What's not to love--for buyers or sellers?

I wish the Internet had an impulse rack. Regrettably, there is not yet any good way to sell cheap stuff on the web. The fees to process credit cards online--an average of 2.2% of the sale price plus another 25 cents--eat up a lot of margin in a 50-cent item. Nevertheless, several startups are trying to crack the puzzle of "micropayments," the geeky term for selling low-cost items. Most of them are doomed. But this is a space worth watching. Once someone gets micropayments right, it's going to offer huge opportunities for small business.

Think about how many things you buy in the real world that cost less than $5. Think about how much expertise you possess that might be worth something to others. (We're talking about digital goods here; shipping fees make low-cost physical items untenable.) "Once a micropayments system gets widely adopted," says Todd Pearson, PayPal's managing director of merchant services, "we'll see businesses selling things that none of us could ever dream of." Consultants could sell PowerPoint slides for a buck. And e-tailers could add an online impulse rack--imagine stereo buyers springing for a video at checkout that shows how to manage that mess of wires.

Two micropayment startups in particular (both of which debuted in December) target small businesses as merchants. BitPass avoids high credit card fees by asking customers to prepay, processing a number of future sales at once. Peppercoin does it by aggregating small payments as one charge on a regular schedule. BitPass collects a flat 15% fee to merchants on items under $5; Peppercoin's formula is more complicated but averages between 7% and 10%.

BitPass and Peppercoin may be friendly to you, but they're not friendly to your customer--and you're going to get blamed for that. Peppercoin forces customers to download software before they can return to a store and purchase what they want. Customers have to buy prepaid BitPass "cards" (from $3 to $60) before purchasing content, usually priced between 10 cents and $1. Imagine going to the five-and-dime and being forced to buy a $20 gift certificate. I thought the idea was to create an impulse market, not scare it away. What's more, you're asking customers to purchase the equivalent of Confederate money--currency that might not be redeemable anywhere else they shop. Until one system wins broad acceptance, you risk alienating customers for good.

If you insist on trying a system, I recommend Peppercoin, if only because its execs acknowledge their product's shortcomings. Keep an eye on PayPal too. It already has critical mass with 35 million users, and its current offer to lower its fees for major music sites (2.5% plus 9 cents for items under $5, vs. its standard 2.9% plus 30 cents) could be a breakthrough if extended to other businesses. Unfortunately, no one expects that to happen anytime soon. Not even Shannon Sofield, founder of PayLoadz, a micropayments system that lets folks pay with PayPal. "PayPal won't give up that 30 cents a transaction," he says. "My feeling is that micropayments are not viable just yet."