Any bus companies from Sofia to Thessaloniki?

There will be several and some lines come from onward or go further, price depend on how long advacne you book and by dayofweek and company-its probably now still far too early for may-and also no need at all for these eternal worries about fullness etc. In fact-to the extreme utter surprise of those grown up with fones-there may even still be the odd comp sending out buses by just selling tix in their office-in front of SOF main train station used to be a large field with shacks just doing that-and the ´official´ bus gara was to the left of it. I estimate a fare would be around 20€.

It is as such, if you want to press max out of that pass to do whole sector till border in BG by train and from a stop some 30 KMs or so onward from where the odd GR trains run again and cover this short sector by bus or taxi-thats for diehards who do not ,mind loosing many hrs doing that. BUt it may be, if you team up with some likeminded passholders in a hoStel (hostelmostel rec.!) in SOF that with a small group that taxi cost less p.p. The bus for it may well turn out to run the whole line Sof-Thess.

But if you have like so many seem to do nowadays a set up plan for daytoday and cannot stand even the thought of having something to rearrange, then check what for next week or so comes out of google-note that and try that a month advance or so before the planned date

More info also on seat61.com, always promoting train travel to the last drop