The checks are in the mail, the old saying goes. Only this time it’s true.

Checks, thousands of checks, millions of checks, all blank and ready to be filled in, are pouring into the mailboxes of consumers all across the country. They are coming from credit card issuers, hungry for new interestbearing business in an ever more competitive market.

No one knows exactly how many are going out, but these offers to advance cash have become an important component of the 1 billion solicitations sent out by credit card issuers annually.

“It’s a sign of a competitive market,” said Robert McKinley of Ram Research, a Frederick, Md., firm that follows the industry.

The pitches vary, but all are tempting: Take a vacation. Pay off holiday bills. Grab an advance on your tax refund.

No matter how enticing they sound, however, experts say there is one important fact to keep in mind about these offers - they are loans, and you will have to pay them back, very likely at a stiff interest rate.

“Be aware just how costly - costly is an understatement - these cash advances are,” said Ruth Susswein, head of Bankcard Holders of America, a consumer group headquartered in McLean, Va.

Figuring that out may not be easy, she added. Much of the promotional material accompanying these offers is misleading, she said, and the interest rate isn’t the only way issuers can add to the cost.

Susswein, McKinley and others cautioned consumers to examine any solicitation very carefully, and perhaps call the issuer for clarification, before using one of these checks.

They suggested these steps:

Check the interest rate.

Don’t assume that the rate on these checks, which are known as cash advances, is the same as the one charged on normal purchases.

Take a look at the disclosure box - typically a little block of small type with a box around it. The rate may be there. If it isn’t - and McKinley noted that issuers don’t have to include the rate on cash advances in the box - call the issuer and ask.

Make sure also that the rate quoted in the promotional material or over the phone is the real one. Most credit card rates are variable these days, but many issuers are offering temporary promotional rates that click off after a period of weeks or months.

Check the transaction fee.

Issuers often charge a fee for a cash advance. Usually this is a percentage of the advance - 2 percent or so - but with a minimum and perhaps a maximum, as well. Typically these range from $2 to $10. That may not seem like much, but on a small advance over a short time it can work out to a hair-curling annual percentage rate.

In fact, issuers are required to show those charges and treat them as if you would be paying that level of interest all year, Susswein said. As a result, Bankcard Holders has been getting calls from stunned members who have gotten statements showing them paying 36 to 39 percent.

Check the grace period.

Many credit cards do not begin charging interest for 25 days or so if there is no outstanding balance on the card. This allows convenience users, those who pay off their balances in full every month, to use their credit cards for free (assuming their card has no annual fee).

However, the interest meter starts ticking immediately on most cash advances.

And one other caution: If you get a bunch of these checks in the mail and don’t plan to use them, destroy them. They are negotiable, and while you can get off the hook if someone forges your name, you don’t want the hassle.