Commuters might want to start swiping their MetroCards a little more gently.

On top of a fare hike that went into effect Sunday, the MTA is now charging riders an extra $1 for buying a new MetroCard instead of refilling an old one, saying it will help clean up stations and save money by printing fewer cards.

But straphangers and their advocates say the so-called "green fee" may face some hurdles if it is to achieve anything beyond bringing in an estimated $20 million into the cash-strapped agency's coffers.

"The idea of getting people to reuse their cards is good, but it ends up being a burden on people," said Bill Henderson of the MTA's Permanent Citizen's Advisory Committee. "It's another increase in the cost of riding."

While it has been possible to reload extra cash onto regular MetroCards for years, unlimited weekly and monthly cards could not be refilled until last year.

Since then, less than 13 percent of straphangers have bothered refilling the time-based cards, instead preferring to toss them and get new ones. MTA spokesman Kevin Ortiz said the agency expects the surcharge to change that statistic.

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"Once the $1 surcharge kicks in, folks will definitely gravitate toward refilling those cards," Ortiz said.

It's the MTA's responsibility to get the word out to riders about the new fee, said Gene Russianoff of the Straphangers Campaign. He likes the fee because it could reduce waste.

"It's something new. My guess is it will have a bumpy start," Russianoff said. "I'm sure a lot of people don't even understand that . . . [MetroCards] are refillable."

Because MetroCards have an expiration date of about two years from their purchase date, riders are going to have to be more careful to avoid damaging their cards -- especially those that swipe on a daily basis.

Catie Monck, a publicist from Bay Ridge, said she frequently gets an error message from the MetroCard she refills -- particularly when she rides an older bus.

"The bus driver will do the 'just rub it on your leg and try again,' " said Monck, 36. "But more often than not, it just reads 'error.' "

She called the new $1 fee "ridiculous."

"The cards aren't 100 percent effective, obviously," she said. "You shouldn't be charged to get a new card."

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Out of about 2.6 million cards sold each month, about 30,000 are sent back in because of damage, the MTA said, with unlimited monthly passes making up about half of all returned cards.

Another 4,200 expired cards are sent in to have their value traded to another card each month.