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Buying your groceries online can be a huge time-saver, but a new study reveals just how much you're paying for that convenience. According to a survey of prices conducted by Anthem Marketing Solutions, online grocery shopping carries a premium of about 25% over its offline counterpart.

In Anthem's survey — which includes categories ranging from personal care to electronics — about 75% of prices were the same. The remaining 25% of prices were almost universally cheaper online, with one glaring exception.

While online shopping has driven down the cost of nearly everything else, the grocery category remains an outlier. "In this category, online outlets offer, on average, prices 24.6% higher than at offline retailers," the report said. It compared prices at Amazon, Netgrocer and Peapod with several traditional grocery retailers, including Jewel Osco and Walmart.

In the results, only Amazon had lower-than-average prices--an impressive 18% lower, in fact. The site didn't beat Walmart in terms of cheap prices overall, but it did do better than offline grocery retailers Mariano's and Target. Prices at Peapod came in about 6% higher than average. The online grocery service that really tipped the scales was Netgrocer: Anthem's survey found that its prices were 43% higher than average. (None of the results included sales tax or shipping since those vary so much by location.)

If nothing else, the study should demonstrate that when buying groceries online, it's essential to shop around. Compare prices with other online grocery services--and with in-store options as well.