Hi Ivana,
Combining many orders into one invoice has many complications. For example: if Order 1 has 5% discount applied to entire order, and order 2 has no discounts, then merging creates calculation uncertainties. None-the-less, there are other factors to consider, such as due dates differences, billing/shipping addresses differences, tax-exemption differences with orders, etc. Thus, I'd not recommend going that road.
However, there's a solution. I believe what you are looking for is 'customer statement'. This can be access through:
- 'View->Customers'.
- Select your client and click 'Statement' on top toolbar. You could preview or print it.
- It contains the summary of all the invoices, amount due, amount paid, for the selected period. You could send this to your client. Make sure to verify and match all information on the statement with invoices and your expectations before sending it. 
Hope this helps!