I list CPC and ERPA. The CPC implies the other ASPPA designations were achieved.
I add then for a few reasons. One, it implies some sort of industry competence, even if the layman has no idea why it's there.
It's there to show other professionals in the industry that I've gone through the rigours of the training.
It's good for potential employers to see.
From time to time a client will ask me what the letters are for, and I'll tell them. It helps garner some trust in my abilities.
If/when I get the Tax exempt/Govn't plans designation, I'll add that in there. Maybe. Not sure yet. At least on these boards here and on my resume I'll add it. It shows people that I might know what the hell I'm talking about.