More than 1.5 million individuals hit by heavier-than-expected 1976 tax bills would get sizeable refunds under legislation cleared this week by Senate-House conferees.

The tax refund provision, for 1976 income on sick and disability pay, is one of the amendments Senate-House tax-writers have agreed on as part of President Carter's economy-boosting tax bill. Both the Senate and the House are expected to approve the compromise, and the President to sign it.

For persons with 1976 income for sick or disability pay, it would mean they could exclude from U.S. taxes amounts of up to $100 a week for that kind of income. The exclusion would not apply to sick or disability income received in 1977 unless the individual could prove he or she is permanently and totally disabled.

It is estimated that more than 1.5 million people each year took advantage of the $100 a week tax exclusion break on sick and disability income. Because the federal government has been generous in permitting employees to retire on disability, a large proportion of those using the tax break are, or were, federal or postal employles.

Late last year, Congress - apparently by mistake changed the law retroactively by eliminating the $100 a week sick and disability pay exclusion as of Jan. 1, 1976. That meant that many individuals who had planned on smaller tax payments had to come up with extra money, ranging in some cases from $60 to $1.900 or more.

The Senate and House (pushed by Sen. Robert Dole (R-Kan.) and Rep. Bob Daniel (R-Va.) tried to correct the retroactive tax change before the April 15 deadline, but did not make it.

The corrective action was not taken because House objections to unrelated legislation tacked on to the tax break bill, and a warning by Senate Finance Committee Chairman Russell Long (D-La.) that President Carter might veto the package.

At any rate, Senate and House tax experts appointed by the leadership have agreed to the tax plan Carter wants - with the amendment that would change the effective date involving sick and disability pay exclusion for most individuals from Jan. 1, 1976, to Jan. 1, 1977.

Unless a new roadblock pops up in the Senate and House, quick clearance is expected and President Carter is expected to sign it.

When (and if) that happens, individuals who paid out extra money on 1976 income derived from sick or disability payments will be able to file for refunds.

Many married couples who must file jointly (thereby boosting income) will not get the tax break, nor will persons over 65 who come under the new (1976) tax code provisions. In every case, people who think they have money coming from the government in the form of refunds for taxes paid on sick and disability income ought to cheek with the Internal Revenue Service or their accountant to make sure they qualify for the benefit and follow the correct procedures to get it.

Post Card Onslaught: Mail volume for the 9-cent cards is expected to jump in the next few weeks as more than 200,000 union postal workers write members of Congress asking them to support Hatch Act reform and labor-management legislation.

The Hatch Act changes would permit federal and postal employees to take active roles in partisan politcal campaigns. The labor-management bill would set up a legal system for handling in-house government manager-union affairs that is now controled by executive action.

To make sure Congress gets the word - to vote "yes" on both propositions - the American Postal Workers Union will supply each of its 230,000 plus members with postcards (and free 9-cent stamps) already addressed to their representative and senator.

If everybody does his or her writing assignment, that will result in more than half a million postcards hitting the Washington post office and Capitol Hill mailrooms within the next few weeks.