I am with you. If it worked all this time, why need a booster pump. I have that pump in my home because my condensation line ran in my slab and the slab cracked and broke the line, therefore I installed the pump and the condensation dumps in the pumps resovior and as the float dictates, it dumps into my pipe vents in the attic.
Now, a few basic things to consider. The HVAC unit has two pans, one primary and the secondary kicks in when the primary rusts out. They are stacked under the unit and catch that condensation drips and all the pans are set at a slight angle and the water congretates at the lower end. There is a pvc pipe attached that then carries the water to the outside.
Several things can happen. Algea, moss, growth or whatever has clogged the pvc pipe. you can blow this out with water. To kill the bleach, you need to periodically pour bleach into the pans to clean out the stuff and allow for nice smooth flow. How often do you do this? Could I speculate never?
what I would do, is before I throw any money at this cause, I would go up in the attic with a flash light and be careful NOT to fall through the ceiling. Look at the pans and look for debris and a build up of algea. Then pour a gallon of bleach/water into the pan and have someone on the outside pvc line wait for water to exit.
But, back again, if you have the pans overflowing, you need to get a coat hanger and prod around at the pipe and see if you can obtain a water flow. Do this first as adding extra water will only worsen the problem.
If you see that both pans have a rusted hole in them then you have a big problem. I have found that the labor to replace pans is almost as much as to replace the entire upstairs system.
If no holes, then get by whatever means avail a water flow to the outside. When you do this the unit will work again.
Now, forget the all you need is a pump man/woman. This person might be taking advantage of you. Do your work and then get three more reputable bids if needed.
You also have to add bleach to the pump if you have to have one.