I bought the domestic rate book today, but it has been killing me to know if this is paying a specific rate. Might you indulge? This postmark/stamp are on a #10 envelope. The only postal marking is a small purple C4 on the back.
I am a specialist in the Americana Series. The 28-cents paid the two-ounce domestic letter rate in the 15-cent era (15c + 13c (for each additional ounce) = 28c) from 29 May 1978 to 21 March 1981.
Hope this helps!
David
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"President, The Society for Costa Rica Collectors"