Amazon may increase its US Amazon Prime membership fee by $40 dollars more, the company’s chief financial officer announced Thursday during an earnings call. CFO Tom Szkutak said growing transportation costs may cause the online retail giant to add between $20 to $40 to the subscription fee. Prime — which offers free, two-shipping on certain items and access to Amazon’s Prime Instant Video library and Kindle lending library — currently costs $79 a year. He said the subscription service, has become very popular in the last nine years. Prime shipments have grown from 1 million to 19 million units, according to Szkutak. “Shipping costs have gone up a lot, fuel costs have gone up a lot,” he said. Szkutak said the Prime program has “tens of millions” of members worldwide, but he wouldn’t say what the figure was for US customers.