Costco Poster Print resolutions

What resolution is needed for a 24 x 36 in poster print at Costo.

I cropped one 24 x 36 at 300ppi, 10800 x 7200 but was told the resolution was too high..

Earlier by mistake another cropped at 11.5 x 19 in at 300ppi 5700 x 3450 was given to them (looking for a 24x36 and was told that the resolution was too small for a poster print. Although the format was incorrect this would work out between 140 and 160 ppi.

I always crop at 300 ppi for printing.

Anybody know what they require.
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Old Sea Dog
Galleries at http://www.pbase.com/oldseadog/galleries