U.S. Postal Service payroll costs will jump about $275 million this May when more than half a million rank-and-file workers get inflation-triggered raises worth nearly $10 a week. The cost-of-living boost is the first of three pay raises guaranteed workers of the federally owned mailmoving corporation in 1979.

If inflation continues at the current level, the impact of the three raises could approach the $1 billion mark, and deal a real blow to the administration's anti-inflation guidelines. USPS officials say the first boost will push average pay for postals to more than $17,000.

The upcoming increase automatically goes to clerks, letter carriers and other nonmanagement personnel covered by union contract. It is the first of two COL raises for the year, and is in addition to a July straight pay raise for postal workers of about 3 percent. The second living cost boost is due in the fall.

Unlike their white collar federal counterparts whose pay raises are controlled by the president, postal workers bargain directly with management on wages and fringe benefits. The current contract calls for three guaranteed pay raises, and six cost-of-living boosts during a three-year period.

White collar federal workers and the half million blue collar (wage board) employes have had their 1979 pay raises limited to 5.5 percent. Increases for white collar civil servants (more than 300,000 in metro Washington) will come due in October.

President Carter already has said he intends to hold them to 5.5 per cent, no matter what the wage gain in the private sector is or what the inflation rate for this year turns out to be. White collar federal workers nationally average around $19,000, with the pay average here about $22,000.

The gap between postal and other federal pay is closing steadily, thanks in large part to the postal workers linkage to the cost-of-living.