The FAQ clarifies that the items will not provide any gameplay advantages.
Payday Credits will be added to Payday 3 after launch. In addition, the game includes a Gold and Silver Season Pass. The Silver pass will grant access to DLC from the first six months, featuring 2 heists, cosmetics, and weapons. The Gold Season Pass includes the first year of DLC, consisting of 4 heists, cosmetics, and weapons. It is mentioned that the DLC model for Payday 3 will be similar to what players are used to in Payday 2 – DLC can be purchased separately.
As for other post-launch content for Payday 3, the FAQ states that the development team is looking forward to sharing details on this closer to the release date.