8 of the best things you can buy at Costco, according to superfans who have been to over 200 warehouses

Before they started buying pesto from Costco, Susan and David swore by making their own at home with fresh ingredients from food markets.

But when they picked up a jar of Costco's basil pesto made with ingredients imported from Italy, everything changed.

"I gave up," Susan said. "This is better."

Besides having an "unbelievable" flavor, the couple said it also "lasts forever," making it a staple item in their pantry.

Costco's Tuxedo-style cakes, which are long and rectangular, are the perfect treat to feed an entire party of people, David and Susan said.

They've tried most of the cake flavors, which include the tiramisu and chocolate mousse, and even bought it for the book party launch they threw ahead of the release of "The Joy of Costco."

David and Susan said the cakes are also perfect for the festive season. "$17 for that cake. Merry Christmas," Susan said.

David and Susan said they recently came across the $8.89 tub of carmelized-onion dip and have decided it's one of their favorite foods sold at Costco.

"It's utterly transformational," David said. The thick sauce can be used as a dip for chips or added to the top of a burger, according to the couple.

"And if you eat the entire thing, it's only 2,300 calories. So it wouldn't even be a diet disaster," Susan said. "This will change your life."

"We have that every morning for breakfast," David said.

"Not the whole bag," Susan added. But the couple said they do find a way to incorporate the real pieces of bacon into different meals throughout the day.

"It's so fantastic in salads, put it in eggs and scramble it. This is a Kirkland Signature item, and it's one of my favorites," Susan said.

One of the myths that David and Susan try to debunk among Costco newcomers is that the wholesaler only sells products manufactured by big businesses.

Throughout the aisles, they said, you'll find products created by smaller businesses and restaurants chosen by Costco to sell worldwide.

One of their favorites is Rao's marinara sauce, which was created by a small Italian restaurant in New York City that Susan said is "very exclusive."

"It's hard to get a table unless you've been going there for 30 years, and then they started selling this commercially," she said.

Pam, a cooking spray that uses Canola oil as its main ingredient, is sold at Costco in packs of two for $6, making one can essentially worth $3.

"We buy all our Pam here because it's like half-price," David said.

"This can would be like $8.00 for a smaller can at the supermarket," Susan said.

David and Susan said they don't usually eat at the food court anymore because of how many times they've had it before.

"Sometimes we do, but we've been to over 250 warehouses so we like to try stuff that's different. In other parts of the world, we'll order everything," Susan said.

Nonetheless, they do believe that Costco's famous $1.50 hot dog and refillable soda combo is a must-buy for regular members, especially those visiting with their families.

"It's $1.50 of joy," David said. "$20 feeds a family of five."

The couple can also vouch for the quality of the hot dog ingredients, having visited the wholesaler's meat processing facility in person while researching for their book.

"If I had to have heart surgery, if it was a Costco facility, I would be comfortable having them open up my chest," David said.

According to the couple, the best place to buy designer glasses for a relatively affordable price is at Costco.

"Costco has such a great relationship with the vendors that they can get these products very, very inexpensive," Susan said.

They also recommend Costco's hearing aids, which they said are much more affordable than other vendors.

"My hearing aids were $1,500 and they would have been $10,000 anywhere else," Susan said.