What I’d love to see is a Sale input option. Let me elaborate.
100 items/services in product list, plus several customers/clients
Today I sell 5 items to customer D
Item 1: 1x
Item 2/option 2: 1x
Item 2/option 3: 2x
Item 3: 1x
In admin, I search for and check the five  items which I sold (with correct option). A drop-down allows me to add these 5 items to the customer of choice.
Another drop-down will allow me to [a] generate an invoice or [b] generate invoice later.
Latter option, option [b], will keep the items in a customer sales “outstanding inventory”. The next sale to the same customer will add the newly sold items to that sales list and accumulate all sales. When ready, an invoice is created.
Like in my case, I have customers on account. They order/buy weekly but get invoiced monthly.
This upgrade would be an enormous feature upgrade to the current invoiceplane.