The Georgetown hostess who favors dainty party invitations had best find a reason to use them by tomorrow. Starting Sunday, she'll have them returned.

That's the day the U.S. Postal Service has decreed it will ban mail less than 3 1/2 inches high, or 5 inches long or .007 of an inch thick.

In addition, it will cost seven cents more - 22 cents in all - to send an oversized or odd-shaped piece of first-class mail weighing less than an ounce, with dimensions exceeding a height of 6 1-8 inches, a width of 11 1/2 inches or a thickness of one-fourth inch.

The reason, according to the Postal Service, is that the tiny mail and the jumbo letters and cards jam automatic mail-processing machines. That mail is often damaged and mail in the machines behind get stuck as well.

For the most part, people buying stationery in the next few days won't have to worry about purchasing thank you notes, party invitations or birth announcements that would be too small.

Most stores have apparently already sold what small envelopes they had in stock months ago or earlier.

The National Association of Greeting Card Publishers, whose 45 members have 80 percent of the U.S. greeting card business, said the manufacturing of the small envelopes has been phased out over the last three years, starting when the Postal Service first announced plans for the ban.

Most large cards already cost 28 cents to mail, according to the trade group's spokeswoman, Jacquelyn Jackson, because they weigh more than the one-ounce limit for a 15-cent stamp.