A 5-cent price increase for the the United States Postal Service's (USPS) "forever" stamp goes into effect Sunday, marking the largest price hike for the stamp since 1991.

The USPS originally proposed the increase from 50 to 55 cents in October, and the increase was improved in November by regulators.

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The 10 percent hike is the largest since 1991, when the price of the stamp jumped 16 percent from 25 cents to 29 cents. 

The 5-cent hike is also the biggest nominal price increase in history. 

Also going into effect Sunday are increases to priority mail rates by 5.9 percent and priority mail express rates by 3.9 percent.

The USPS said in a statement at the time of the recent proposal that the Governors of the Postal Service "believe these new rates will keep the Postal Service competitive while providing the agency with needed revenue."

The USPS reported a net loss of $3.9 billion during fiscal 2018.