Amazon today said it's "considering" raising the price of its $79 per year Prime service to customers in the US by another $20 to $40. The possible increase — which has not been finalized — was announced during the company's fourth quarter earnings call with analysts, and due to higher costs of shipping goods, Amazon CFO Tom Szkutak said.

Prime membership could top $100

Prime was launched nine years ago, and now has "tens of millions of customers globally," Szkutak said. More than 19 million items on Amazon are prime eligible, something that gives subscription buyers free two-day shipping. Amazon's also added to it beyond product shipping by giving Prime subscribers free book borrowing for some Kindle titles as well as access to streaming movies and TV shows. Szkutak dodged a question during the call about whether any of those services might be broken out into separate and potentially less expensive subscriptions.

The possible increase comes just three months after Amazon raised the threshold for free shipping on product orders from $25 to $35, a move that came without explanation from the company.