Pre-Fabricated Pavilion on sloped concrete

Last year we had a new patio poured to replace a badly damaged one. At the time we weren't sure how long we were going to be in the house so we didn't invest money in a pavilion or pergola at the time, but knowing that down the line it would be a possibility i mentioned it to the crew and since he had a recommended size of pavilion for the size of patio he poured the concrete thicker in the area where we would be putting the posts.

Fast forward to today we are going to be in the house for a while and had the opportunity to buy a pavilion from Costco at a very discounted rate that fit the dimensions we would need. Here is my dilemma. The patio is obviously sloped away from the house so rainfall moves away from the house.

So the question i have is how to install the posts. If i just install them as it is now the posts farthest from the house will be lower than than posts close to the house. If i do this and keep all of the cross beams square with those posts it will be leaning slightly away from the house.

If I cut the posts that are closest to the house down based on how much the patio slants I have to re do all the pre-drilled holes. That doesn't seem overly difficult, but the company specifically says modifying the structure in this way compromises the safety of the structure and voids any warranty. Also when i do this since it is a small slope i would have to either cut the posts at an angle to compensate for that or i would have to shim them.

As for hiring someone to do the work for me, in general hiring someone sounds like a good idea, but the 2 people who i had already talked to may have thought about it and knew they were going to do it, but when i asked what they would do they didn't mention this being a problem at all which worries me that they had not thought about this, or it tells me that maybe i am blowing this out of proportion.