PO Box vs Post Office Address

Well there are several post offices in the Varna region.  If you mean the central post office which is opposite the Cathedral.  The contact details are as follows:
bul. "Saborni" 42, 9000 Varna Center, Varna
Website which has an English button
www.bgpost.bg

Thank you!

So if I put this, along with my name, as a return or delivery address the post office will hold the package for pickup?

(I've heard that mail goes missing a lot. I'm not sure where it goes missing, but I do know it's pretty difficult to run a business if the mail never arrives...)

If you want to pick up the package at the post office, you need to have written on it "„До поискване”   (Do poiskvane) Otherwise, they will just return it. If it has do poiskvane on it, they will usually hold it for 21 days.

Great. Thank you again.

Does it have to be in cyrillic? (The guy at the other end might have trouble with that...)

I have had things saved at the main Varna post office with it written in latin letters, no problem. I've also used "poste restante" successfully (the French, which seems to be used worldwide).