I have to do sth like that:
- improve the database schema design, to allow to store arbitrary number of service/product entries on an invoice.

I have no idea how to do that. Maybe there is any feature in access which will allow me to do that? I have 5 products already and I can input next. Do you have any idea?

You need to remove the product information from the invoice table and create a junction table which you can name Invoice_Products. This table will contain id_Invoice, id_Product, and Product_Count. It will have a 1-m relationship to invoices and a 1-m relationship to products thereby implementing a m-m relationship between invoices and products.