You can’t say American Express, one of the US Open’s signature sponsors, doesn’t give you fair warning about your wallet when visiting America’s grand slam event: Don’t leave home without it.

Before you check out the annual tennis tourney that kicks off on Monday, take a look some of the overpriced merch that awaits you at Flushing Meadows:

Disregard the fact NYC water is one of the cleanest and best-tasting in the country, and make way for the “Official Bottled Water of the US Open” — Evian — with its fancy accoutrements and prices to boot. The facial water spray cools you down after getting heated about coughing up $12 for it.

If you can swallow these prices, you can swallow anything — but make sure you do it in style with this official USTA mug ($20).

If the skies do darken over Flushing Meadows, fear not — this wet-weather essential ($62) has been wind-tested by College of Aeronautics to 55-plus mph.

Kids love to get their favorite players to sign these oversize, 9-inch US Open favorites ($35). Plus it’s pink, so you really know it’s authentic.

Pretend like you don’t have your 20 percent Bed, Bath and Beyond coupon waiting at home, and enjoy a fluffy US Open towel ($36).

The Moët & Chandon Terrace pours out its eponymous Imperial Champagne — because why not go into bankruptcy with a bang!