The testing will start Thursday for customers in Beaumont, Texas. A company spokesman said broadband subscribers will be charged $1 per gigabyte above monthly usage allowances.
The spokesman told Reuters the policy probably would affect the top 5 percent of users who spend a "disproportionate" amount of time on the network.
Customers in Beaumont will be offered different service levels, including a low-end plan that will offer 768 kbps service with a 5 GB limit for $29.95 monthly.
A $54.90 monthly plan will offer 15 mbps service with a 40 GB limit.
Some technology blogs have criticized the plan, saying it would affect customers' interest in watching online videos and anger those who currently pay for unlimited service.