You paid them good money to do a job correctly -- take it back and shove it down their throats.
It sounds like they forgot to install the oil slinger -- a washer/spacer looking part that actually works to sling the oil into the drain channel so it doesn't blow out the seal -- that, or -- they somehow messed up the vent line and it is getting hot and blowing out the seal.
Where is this vent line? And if this washer is missing, how hard is it to replace? Can I just take it apart and put in a new one, or will it need to be dial calibrated or whatever?
I just don't have the tools to really do gear work. This is why I paid these clowns to do it. I've done everything else to this car myself.