Are you sure you are installing "Modulo Bodykit" and not "Modulo Style Bodykit" which is imitation?

Any plastic injection mold are subject to shrinkage after the molding. High injection pressure machine, precise mold and control of even shrinkage is expensive.

That's why LEGO, action figures, Bandai model kits are so expensive.

When comes to bodykit
- Top quality = direct surface sanding and painting.
- Ok quality = touch up, including surface touch up with resin exactly like the yellow/brown clay resin after accident, sanding then only paint. Yours falls around there, a lot of Taiwan 3rd party accessories are in this quality level. Maybe they didn't touch up and straight paint.
- Bad quality = At lot of repair, and high variation between batch. Customer A ok, then customer B fitting problem. Need to cut and retrofit for some case. Some warp not straight. The mold also not accurate, the sharp lines are blur.

These are the molding quality, and not related to the parts durability.
Some shop don't know lazy or no experience, didn't sand and straight paint base coat. Even good part the paint adhesion is not good, will peel off after a few years.
Good workmanship shop will sand, then paint adhesion promoter primer, then only base code, and other layers.

Depends on the paint, my detailer use Glasurit paint, with medium speed hardener on clear coat. He always recommend 2 months before coating or PPF also by him.

Side skirts, diffuser are prone to abuse, after 2-3 months of daily driving the surface will be rough on touch. Depends if you want perfection on the parts that destined for abrasion loh.