I'm very new to Netrunner and am trying to build my first couple custom decks so I apologize if this question has been answered before. The question I have is, with Accelerated Diagnostics (AD), I understand the mechanic of the card and what it does, but am not sure how it affects certain cards. Such as, with Scorched Earth (SE) and Closed Accounts (CA), does that mean if let's say SE and CA are 2 of the 3 cards from the top of my R&D, I can then pay total 4 credits and do 4 meat damage and force them to lose all their credits, even if the runner isn't tagged, since AD says I can ignore additional costs to play? Or is the "Play only if the runner is tagged" considered part of the first initial cost to play the card?

Also, using AD, if Shipment From SanSan is one of the top 3 R&D cards, I'm assuming because it specifically mentions "additional cost" in the text, that this one is a little more straight forward, and that I could play that card and gain 2 advancement tokens without having to pay the additional click cost. I'm just trying to figure out what exactly constitutes an "additional cost" that can be ignored by AD, so I can figure out what cards can and can't work with it. Any input would be appreciated. Thanks.

"Play only if the runner is tagged" is not an additional cost, it is a play restriction and still applies to events found with AD.

Any additional cost is clearly marked by the appropriate language "As an additional cost to play..." which you've correctly identified on Shipment from SanSan. AD would allow you to resolve Shipment from SanSan without paying the additional click.

Appreciate the info