There are plenty of items to load up in your shopping cart at Costco -- but gold bars shouldn't be one of them.

Investors are still buying up Costco's supply of gold bars — and there are a few big reasons behind this trend.

Newmont Corp., one of the largest mining operations in the world, announced early Wednesday it will be minting a traceable gold bar through MKS PAMP, of Switzerland, using Provena

Costco appears to have found a revenue mine with sales of gold bars, as a survey found that some warehouses are unable to keep them on shelves. Bloomberg found that 77% of the 101 stores it surveyed in the first week of October had sold out of the bars, the financial news outlet reported Monday.

The Swiss-made bars are only available to Costco members, whose annual fees range from $65 to $130. Made of 999.5 pure platinum, the bars are non-refundable and limited to one transaction per membership at a maximum of five units per order. They are only available online and can’t be delivered to customers in Louisiana, Nevada or Puerto Rico.

After finding massive success with its gold bars, Costco just introduced 1-ounce platinum bars. Here's what you should know before buying one.

Retailers like Costco and Walmart are selling precious metals including platinum bars, driving a new demand for precious metals.