In eBay's July quarter, management said it expected to do $20 billion of mobile commerce and payment volumes this year. Braintree already does one-quarter of that on an annual basis, the company has a strong list of places where its mobile payments are accepted, including Opentable ( OPEN), RoviHeroku, TaskRabbit and several others. It's also no coincidence that Braintree's impressive growth has paralleled the declines of traditional payment systems from the likes of VeriFone ( PAY) and Ingenico. eBay's management deserves plenty of credit here for seizing an opportunity to build on the cash powerhouse that PayPal has already become. But I don't believe it's time to rest. If Apple ( AAPL), with its fingerprint technology, picks off Square soon, it will be game over for PayPal. As brilliant of a move it was to acquire Braintree, eBay's management will have plenty of other decisions to make. Not the least of those will be which companies it should buy next. Square has to be at the top of the list. As with Braintree, Square makes it easy for small businesses to accept credit cards without installing a high-fee terminal. And given that Square is estimated to process close to $5 million worth of daily mobile transactions, which average close to $2 billion per year, it's another no-brainer. At the time of publication, the author held no positions in stocks mentioned. Follow @saintssenseThis article was written by an independent contributor, separate from TheStreet's regular news coverage.