I wrote a lenghty reply just to have it disappear with an error message... so here we go again:
Nearest airports with flights to Europe are Denver and Minneapolis. Cargo rates are sky-high at the moment because flights are scarce, and this will likely remain until covid restrictions are eased and flights resume. Not going to happen until mid-2021 at the very earliest.
The only airline currently flying from Denver to Sofia with one stop and accepting pets as checked baggage is
Lufthansa. You need to find someone flying from Denver to Sofia via Frankfurt on LH to take your dog with you as checked baggage. This will cost EUR 380 in fees for the dog crate. Roundtrip tickets start at around $650, so you're looking at $1,100 if you'll have to pay for the person's ticket bringing your dog.
Air Canada will also fly to Europe from either DEN or MSP and accept dogs as checked baggage - and has cheaper fees at $270 - but they don't fly to Sofia. Two-airline, two-stop flights are not really a good idea given covid travel restrictions as you will need to go landside and pass immigration/customs at the transit point (likely Frankfurt or Warsaw) to recheck the dog with the airline operating the intra-European segement.Also, there's ticketing policy issues (AC normally does not accept pets with parter/codeshare segments on the ticket, you may have to book separate tickets). Things like these are best reserved for experienced travelers in non-pandemic times.
You can also ask LH directly about flying her unaccompanied as cargo. This will likely be a good deal cheaper than using an agent/pet shipping company. The paperwork is not much more extensive that what you'd need for bringing her along as checked baggage. Call their
Denver office at + 1 800 542 2746 and ask for a quote.
If any of the above will not work out or prove to be too expensive, you'll have no other option than to wait until United resumes their
PetSafe program. PetSafe is the by far cheapest option to have a dog travel unaccompanied. Maybe get in touch with United and inquire...
.