Poshmark Shirt sold does not fit even though it is marked as my correct size. Redwood City California

I order aa NWT Men's Tommie Copper lower back support shirt Size Medium.  All my clothing is size M and I am a slender at 6 feet tall and 165 pounds.  The cost was $67.22.

The shirt I received is marked M, but I could not get it past my shoulders because it was so tight.  Either it is mismarked or their idea of Medium is not what other makers consider it to be.

I requested to return the shirt for a size Large and their response was "Under our Posh Protection policy, so long as the item was listed correctly at the time of sale, we are not able to offer the option of a return.  This includes cases in which the item purchased does not fit properly." 

They offered me a credit of $7.97 in Posh Credit and suggested I try to resell the item.