Well done on doing the planning first!
I'm presuming that products are derived from a table. The forms could then use combo boxes to list the products for entering into a particular client's record in a table.
When you open the form for a particular client, you could use a bit of coding to copy the previous record for the customer, update the date (or whatever) if needed, and then just make appropriate products purchased changes with combo boxes.
However, that is a straw in the wind idea as I'm not 100% sure of what you need in your db.
A VERY good forum (if you don't have enough success here)on Access is available at click here. It needs you to register - but it is free.
I have had lots of help in there. Responses might be delayed because the vast majority of the gurus are in the USA - but, I wish I had 1% of the knowledge on Access that some of them have.