As a hiker, I had a two litre camelbag with water and a small bottle Ardbeg and some secret stuff from Melnik.
Good chance to Shop cheaply is the Travel Free in Sofia if you arrives, just after the pass control, buyed in novembre Ardbeg ten 31,50€ for a litre.
Anyone whos going with cab be careful at Sofia Airport you have to use the desk from Okay Supertrans, no one else.