Online auction house eBay's payment system Paypal can now be used for purchases by US customers of Apple's iTunes music store.
Paypal positions itself as an online currency
Other download services already accept Paypal but the tie-up with the market-leading iTunes is seen as important.
"This is a big win for our off-eBay business," Paypal's spokeswoman Sara Bettencourt said.
About 70% of Paypal transactions stem from eBay but the firm is increasingly winning new deals with retailers.
For some smaller merchants with an online presence, it is their only accepted method of payment.
The business, which has more than 56 million registered customers, makes money by taking a cut from each transaction it processes.
Paypal sales reached $166m (£87m) between July and September, a 56% increase on the same period in 2003.
But the service, which was acquired by eBay in 2002, has not been without hiccups in the past.
In October it suffered from service failures after a software upgrade.
And in its early days it was accused of overstating the level of protection it offered in the event of non-arrival of goods.