Carrefour is an excellent hypermarket with electricals / food and lots of deals to be had, constantly. 9 day returns policy which is generally unheard of in Bulgaria.
Metro cash and carry (aka Macro in the UK) - the one on Tsarigradsko Shose is open 24 hrs a day, making it ideal to stock up after arriving @ Sofia airport (and e.g. before heading off to your favourite holiday destination, regardless of time of arrival). You do need to set up an account with them though and it does need to be as a business, not a consumer.
Check out Ilienzi market in Sofia. Good place to meet importers of goods / local manufacturers as they have stores set up there. You can quite comfortably talk bulk discounts with them direct.
ilianci.com/
Praktiker also has a good returns policy (a german DIY store).
Technomarket Evropa - appalling - no returns / very difficult exchange policy, even for goods sold as faulty!! They will try to give you the runaround from the get go, persistently denying you your consumer rights. Lots of choice but Technomarket WILL completely deny you your rights and will do so as rudely as possible. Staff will not deluge their names and will hang up on you. I did eventually manage to put them in their place, only to witness staff continue telling customers completely false information in regards to their rights when things go wrong;
"Don't come here - take it to an authorised repair center, even if you just bought it from us."
Prepare to do serious battle with them if you have any problems and want them to take care of it (as they should, by Bulgarian / EU law).
As a general rule, know your EU rights in Bulgaria as shops aren't as kind as UK retailers. Some do have their business hats on and value their customers, but it can be a mine field. Shop online as you get 7 days to return goods (you do NOT have this right if you shop at a bricks and mortar store).
As a general rule, your consumer rights entitle you to 2 years' warranty, regardless of what you are told by the retailer. This funnily enough also applies in the UK (being an EU member).
dolceta.eu/
in Bulgarian only form;
kzp.bg/index.php?mode=viewd&group_id=4&document_id=120
The body which enforces your rights. Very friendly and will consult you over the phone (in Bulgarian, don't know about English). They do swing into action even just using a well written cc: letter / email and will give you an official PDF in response after their investigation is complete.
kzp.bg/
The ombudsman that takes care of business if the above fails;
ombudsman.bg/
The Bulgarian alternative to Which? magazine, entitled 'Active Consumers';
aktivnipotrebiteli.bg/