There is no standard collective noun for 'envelopes', in which case a noun suitable for the situation can function as a collective noun; for example, a box of envelopes, a pack of envelopes, a stack of envelopes, etc.
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Envelopes is correct.
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Envelopes come in several shapes and sizes. I assume Hallmark
envelopes can be printed in any printer designed and capable of
printing envelopes.
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there is NO antonym for envelopes due to the fact that it is a
noun.