Is it possible to get a flat shipping offer on website or its not worth it?
to my knowledge they do not offer flat shipping at all.
They dont offer it because of how much they drop ship. More and more online retailers are going to drop shipping. PSA works on a volume model over a profit model.
well said Guitar Guy
In my opinion PSA manufactured items should be eligible for a flat rate shipping if (in the same order) they are coming from the same warehouse I.e. two AR complete lowers are $17 each or two Dagger slides are $9 each.
I also completely understand the volume business model, so it may not be an option.
Just asking for Admin to run it by the chain of command 
I get it. But with as big as PSA is, not all products are made at a single place. PSA has multiple machine shops in West Columbia, Summerville, and even Jacksonville, FL. All that product has to be delivered to the shipping/building warehouses. PSA does all that on their own trucks. They arnt passing everything off by any means (for example ISP to Fernandina is shipped free and transfered for free) but they cant eat the entire cost. As i pointed out above, drop shipping is a part also and that is its own animal.
i get it too, and if memory serves this argument had gone on on the arfcom in their industry forum over the years repeatedly. It hasnt changed yet, so i wouldnt expect it to anytime in the near future sadly.