You are probably not aware, but EOTech is refunding customers for the return of their units. It is part of a settlement, a recall of sorts. No receipt is required. I just returned a used 552 like yours and received a check for $600.79. They refunded MSRP of $555 plus 8.25% for taxes for a total of $600.79. They also added $15 to cover my shipping costs.
There are many individuals purchasing used Eotechs and returning them for the full refund. Good work if you can get it, I guess.Eotech refund page
Sight Detail Refund
Refund Request Amount
Description of Failure