@Manuel Moroni Thanks for reaching out. You are correct the "PostgreSQL" connector in Logic Apps does not have built-in support for the GROUP BY and SUM functions. However, you can still perform these operations by using a SQL query in the "Execute a SQL query" action.
please feel free to leave your feedback at https://feedback.azure.com/d365community/forum/cb47c115-7926-ec11-b6e6-000d3a4f032c
let me know incase of further queries, I would be happy to assist you.