Undated (AP) _ Postal carriers reported bulging bags in Boston, Little Rock and Los Angeles this week, and sales of two-cent stamps were booming as the days of the 20-cent letter dwindled to one. But some Post Offices reported barely a ripple of anticipation.

First class postage goes up to 22 cents on Sunday.

In Los Angeles, mail volume increased this week by more than 100 percent, and postal workers were working four hours of overtime on Saturday to keep up with the anticipated load.

''We're noting a big increase since Wednesday, mostly in business mail, what you would refer to as junk mail,'' said Larry Dozier, public relations specialist for the U.S. Postal Service in Los Angeles.

''The letter carriers going out have been delivering a lot of mail, so it seems like a lot of businesses are trying to capitalize on the current rate,'' said Lisa DeVincenzo, a U.S. Post Office spokeswoman in Boston.

One downtown Boston post office sold 80,000 two-cent stamps this week, she said, compared to 5,000 in a normal week.

''But we can handle it,'' she said. ''There are a lot of stamps in Boston.''

''We almost have more mail than we can handle,'' Don Jones, postmaster in Little Rock, Ark., said by telephone Friday. ''I think our mail volume this week is a good 60 percent above the same week last year.''

Jones theorized that the imminent increase in rates prompted more sweethearts to exchange Valentine's Day cards. ''We don't count Valentines, but the mail looked redder this year,'' he said.

Darrow Andrews, public affairs officer for the Chicago Post Office, said: ''In the last two or three months, people are doing a lot of mailing. There are some people who have decided to get things mailed a little earlier to save.''

Andrews said businesses that mail advertising at bulk rates are responsible for a large part of the recent increase.

However, there's been no rush for new stamps in Chicago, he said.

In Helena, Mont., postal officials notice no increase in mail volume.

''We've sure been busier at the windows. Some people are buying the 22-cent stamps they know they will need for mail next week and others are getting two- centers so they can use up their 20-cent stamps,'' said Jim Squires, manager of customer services for the Post Office in Helena.

Margot Myers, spokeswoman for the Post Office in Phoenix, said there had been a big increase this week in bulk mail.

''A lot of business mailers are trying to get their mail out before the increase,'' she said. ''As far as regular first-class mail, there hasn't been any kind of dramatic increase other than we normally would get from Valentines Day.''

The Post Office in Atlanta, however, reported no increase in mail volume.

''There's been a slight increase,'' said Ernie Swanson at the Seattle Post Office, ''but it's not like Christmas or the IRS deadline.''

For big mailers, there was plenty of incentive to mail early. Charles Reardon, a spokesman for New England Telephone, said the cost of mailing a telephone bill will go up only one cent, because that is pre-sorted mail. But for a company serving 4 million customers, he said, that works out to $480,000 a year in extra postage costs.

Yet New England Telephone couldn't beat the deadline.

''How can you send a customer's bill out early? You can't,'' said Reardon. ''Customers are billed on cycles. That's the cost of business.''

Effective Sunday:

-First-class first ounce rises from 20 cents to 22 cents, but the charge for each additional ounce remains at 17 cents.

-Presorted first class rises from 17 to 18 cents, and for mail sorted according to carrier route from 16 cents to 17 cents.

-Post cards increase from 13 cents to 14 cents.

-The minimum charge for Priority Mail rises from $2.24 to $2.40.

-Express Mail rates increase 15 percent.

-Parcel post rates will rise by 11 percent.

-The fourth-class book rate will increase 8 percent.