Instant bank interest, like instant pudding and instant coffee, soon may become part of the American way of life.

Instead of having to wait until interest is earned, bank savers can now collect it immediately under a plan announced recently by Washington's Bank of Columbia for depositor's with $1,000 or more.

But the attraction isn't money. For people with large amounts of cash to spare, the instant payoff comes in the form of merchandise ranging from a Mercedes Sport Coupe or a Winnebago motor home to a trip to the French Riviera.

The Mercedes 450 SL Sport Coupe, for example, is being offered in lieu of $27,000 in interest payments to depositors willing to plunk down either $400,000 for a seven-year certificate.

If you only have $320,000 to spare for a one-year certificate, or $55,000 for seven years, Bank of Columbia can get you going on your vacation this summer in a Winnebago Chieftain.

A one-year, $52,000 savings certificate or a $7,800 seven-year certificate is enough to get the bank to organize a $3,000 trip to the French Riviera.

The Columbia bank offer went into effect earlier this month, and so far there haven't been any takers. Columbia officials said there have been inquiries, however, from as far away as New York and Florida.

The latest trend in bank gimmicks to attract depositors is not limited, however, to fancy cars and vacation trips. In Maine, for example, one bank blatantly advertises "loophole certificates" in which it gets around federal limits on interest payments by loaning depositors enough money to qualify for higher interest payments.

In the Bank of Columbia plan, if a customer withdraws the money prematurely, the interest is reduced to the passbook rate minus a penalty.The original price of the car or recreational vehicle is subtracted on a pro-rata basis - giving the customer a smaller amount of principal upon withdrawal. Neither appreciation nor depreciation is taken into account. The same rate of interest holds whether it is paid in advance or after it is earned, although the amount of money is larger later because the principal was not discounted.

According to Columbia spokesman Ronald Richards, the $27,000 (or the Mercedes) works out to an annual interest rate of 7.5 percent for deposits over $100,000 and 6 percent for deposits under the amount.

Currently, there is no federal ceiling on interest for deposits ove $100,000 and the market rate is about 8 percent. Banks may pay up to 7.5 percent on time deposits of six to seven years. The interest paid in advance, whether as cash or merchandise, is fully taxable that year regardless of the length of the certificate.

Advance payment of interest is a cyclical promotional tool, according to an official at the Federal Reserve Board. At the moment, very few banks are doing it, but the practice may catch on. Savings and loan associations are prohibited from paying interest before it is earned.

Instant interest, as cash or merchandise, should not be confused with the promotional gifts banks and thrifts are allowed to give away when an account is opened or increased.

Perhaps the deal being offered by the Casco Bank and Trust Co. of Portland, Me., should be called Super Interest. The bank has elected instead the term "loophole certificate," much to the chagrin of the Federal Deposit Insurance Corp. Nothing in FDIC regulations specifically addresses its legality.

The "loophole certificate" is a variant of the money market certificate, with $10,000 minimum deposits keyed to Treasury bill rates. In short, the bank is offering to lend anyone with at least $6,000 another $4,000 so that the customer can receive the higher interest rate paid on money market certificates.

The normal rate of interest on $6,000 is 5.5 percent. Even by charging one percent interest on a 4,000 loan, secured by the $6,000 deposit, the bank is able to pay a higher rate of interest, now about 7.4 percent, on the entire ($10,000) certificate than it would on the $6,000 alone.