I ran into one issue on pvc pipe, the largest the plumbing supply had was 10" and several piling butts exceed that. In addition one length of 10" was >$250. Couplings were >$50 would guess end caps would be comparable. That comes to over $100 per 6' piling and I am doing 13.

Additional research indicates that fiberglass reinforced plastic has a tensile strength of 20,000 pounds per square inch. A few wraps of fiberglass cloth tape around the joint with epoxy should have the joint exceed the strength of the piling.

I think that will work best for me.