UPDATE: Seems that ASTAK closed the PO box that was listed on the rebate form for you to mail the form back too.
I spoke with an ASTAK employee by the name of Ruby who requested I mail the form to the company address instead. So I will post that address here just in case others are going to mail them in, or if you get the "return to sender" letter back like I did.
mail to: Astak EZ Reader Rebate
1911 Hartog Dr.
San Jose, CA 95131
Since I live in San Jose, and work ~15min away I'm going to just drop off my returned letter. Hopefully it doesn't just get "lost in the mail" from this personal delivery.