Your database seems simple enough, but there's a few things I don't understand.
1. Only one item goes to an invoice? That seems different from every other invoicing system in the world. Usually a system like this has 4 tables--Clients, Items, Invoices and InvoiceItems. That way an invoice can have multiple items and they go into InvoiceItems. So, is it accurate that an invoice will only ever have one item with it?
2. You mentioned services in your question--why? Further you said there were 2 services, but from the drop down I see 5. And you attach those to the Client table and have that figured out already--so why is it mentioned in your post? Lastly, what is a service? How does it differ from an item and is it accurate that a client can only have 1 of them?
3. Why do you have a save button on your forms? It's a bound form as soon as I type something in a field the record gets created and saved. I see there is no code behind the button, but why the button at all.
4. Why do you want to allow deletions? You also have delete button on your forms without code, but I assume its coming--but why? Clients are going to get assigned to an invoice and so will items, if you allow users to delete Clients or items it is very likely you will end up with Invoice records that don't have matching item or client records because people came along later and deleted them. Instead of deleting data it is often best to flag it as inactive. So instead of allowing deletions add a Yes/No field where users can mark a client or item as inactive and have it be removed from further use that way.
