Your tips and tricks for getting the most out of a membership with the beloved warehouse club. When I recently wrote about the cultural phenomenon of Costco fandom, I fancied myself a wizened warehouse wizard. I fell in love with the retailer the same way that Hemingway once described a character going bankrupt: “Gradually, then suddenly.” Now, I own head-to-toe Kirkland outfits and have alienated friends with heroic retellings of deals scored and food samples consumed.

But after receiving several hundred reader responses, I have been very deeply humbled. It turns out that I’m an amateur compared to devoted Costco heads like Jason Methner of San Diego, who extolled a 64-ounce jug of mayonnaise he bought (“more than a human needs”), and Anna Broadway of Anchorage, who regaled me of an 11-pound bag of turmeric that she nabbed on sale. Here are some of the hacks, quirks and unexpected gems that readers shared for getting the most out of a $65 annual membership.

Part of how Costco lures shoppers to spend big is by negotiating steep discounts on luxury items, from vacation packages to jewelry. As a shopper whose splurges occasionally extend to discounted gadgets and hot dogs, the Costco faithful have implored me to think bigger. Much bigger.

By that I mean buying gold bars (roughly $4400) instead of golden raisins, and fine wine ($399 for six different Bordeaux vintages from Château Giscours) instead of green grapes in 3-pound clamshells. Or purchasing diamond jewelry, which several readers extolled. Victoria Summers from El Dorado Hills, Calif., told me that her husband, Rob, surprised her with a three-carat diamond band for their 30th anniversary.

Along with some cat food, Greg Werhane of Colorado Springs, Colo., bought a wedding ring (ranging from $274 to $10,999). When Gordon Russell from Vero Beach, Fla., had a diamond from Costco appraised, he said that the jeweler thought the stone had come from Tiffany & Co. In addition to luxury wares, shoppers can often find special prices on pianos, saunas and even cars with prearranged discounts for members. Mary Coles from Raynham, Mass., bought her 2016 Subaru Outback through Costco after calling other dealerships to comparison shop. “They could not match the price!” she said.

Sal Fragapane from Charlotte, N. C., spruced up his landscaping with a couple of live fig trees (about $69 for a pair). “One of my pride and joy purchases,” he said. Scoring big-ticket items reflects the treasure-hunt aspect of shopping at Costco, where specialty merchandise at reduced prices is known to shuffle in and out of stock.

That’s why shoppers say to always go with an open mind—and budget. Brian Thompson from Prescott, Ariz., recalled the time that he “went to Costco to buy toilet paper and left with a trampoline I didn’t know I needed.” Another reader wrote to me about how they once walked into a store to get a rotisserie chicken and walked out with a shed. There are more than 650 Costco warehouses in the U.S. alone.

According to company practice, as much as 40% of any given store’s inventory can be specific to that location. That means no two stores are the same and that all cater to local communities and calendars. This goes way beyond selling Buffalo wings ahead of the Super Bowl. Émilie Rasmussen of Austin, Texas, praised the shrimp chips that she has often picked up around the time of the Lunar New Year as well as the “top-tier” poutine sold at Costcos in Quebec.

Whenever Lori Ott from Saltsburg, Penn., visits family in Hawaii, she always tries to stock up on Japanese rice crackers and macadamia nuts. Hawaii’s Costcos were praised by a staggering number of readers, most of whom were out-of-staters who found the prices more reasonable than those at other retailers in the state. There is a particular fondness for the Costco on Oahu, regularly cited by readers as the busiest location in the world.

A. Hazelwood Mc Murry from Sacramento, Calif., described it as “akin to the retail version of a pilgrimage to...