May not even... An adverb should go after a subject or an auxiliary if there is one, preferable an auxiliary of tense not aspect. So here it's after "may not".

I would use the verb send or receive, not post. Both of the following options could work~ They may not have even sent (or received) your parcel yet. (This sounds much better to me.) They may not even have sent your parcel yet. (This is also acceptable.)