It’s the internet’s fault that, if the U.S. Postal Service has its way, you won’t be getting letters delivered to your mailbox on Saturdays anymore. After all, how many stamps have you bought lately? But the engine of the USPS’s demise could also save it. After all, how many packages have you received from Amazon lately? And wouldn’t you like to get them faster? Postal Service spokesman John Friess says that while its letter-carrying business is shrinking, its package-delivery business is growing. (Note that postal carriers will still be delivering packages on Saturdays.) In its anxious search to make money any way possible, the agency late last year began testing a same-day delivery service in San Francisco for online purchases. While the experiment, called Metro Post, is starting out small, it also comes at a time when the world’s largest online retailer might be looking for a lot of trucks and a lot of drivers who can do same-day delivery cheap.