At this time you can't merge multiple invoices into one invoice, you would need to combine them manually. Since you have already created all the line items / products / services on the other invoices, they have saved to your products and services library and it's super easy to add them all on a new invoice.
What we would recommend is that you open up a new browser tab, navigate to your Ivy account and use your largest invoice (largest in terms of number of line items), and then add the other line items to that invoice. You'll simply click to add another line item, and the products and services side-bar will pop out on the side, which contains a list of all the products and services you previously created. You'll select the one you want, make sure the quantity and mark-up are set, and then continue on with adding more line items. The item title, description, and cost per unit will carry over from your saved item.