Thanks a lot for your answers.
Some questions:
- I think UPDATE, DELETE or INSERT instructions are not allowed by SAP in POST transaction notification store procedure.
- I want to make an AddOn for controling sizes in products. Products will have sizes (S, M, L and XL for example). Then, when a purchase, sale or transfer stock is done, I would like update stock sizes automatically as part of the general transaction. I could calculate stock sizes with formatted searchs adding purchases and sustracting sales, but I think this is not the best solution for performance having a lot of transactions in database.