Buying and selling is getting a whole lot more mobile.

Verizon Wireless has announced a partnership with American Express that will let users pay for things with their smart phones or tablet computers. According to the San Francisco Chronicle AmEx will begin integrating its Serve mobile-payments system into many of Verizon’s mobile products. This comes after Google and Sprint launched Google Wallet a couple of months ago.

It’s early to know exactly how interested US consumers are in buying things with their phones–but it is clear they’re happy to take payments that way.

Square, the technology that plugs into mobile phones and allows owners to accept credit card payments is growing fast. Tech Crunch reports the company says it now processes nearly $4 million payments every day. The COO expects business to be at $8 million in October.