PO number on invoices

For some of my clients I get a different purchase order for a each, or a number of tasks - rather than at a project level - this is normally part of an ongoing web development project i.e. i'll do the base website and then they'll come back with tweaks and changes before we go live i.e. wanting "Widget X added", I'll quote, then they'll issue a new purchase order...
Ideally I'd like to invoice all of this in one lump, quoting the PO numbers against each invoice line... any chance of something like this being included?
Thanks!!

I'm in agreement with djwhisky that it would be better for me to have a PO number per invoice than per project.
Most of our projects are long term , such as 'website updates' and we would get a new PO for each bit of significant work we carry out.
It seems like more overhead to create a different project each time, although this would work.

Although, I have just thought of a better workaround - use the comments section of the Invoice to write the PO number.

Another shout for P/Os on a per-invoice basis. We're in a similar position as the other users, so would love to know if it would be possible to add Purchase Order refs to the Invoice rather than the Project?

Yup, me too. It's a pain having to create a new project for each PO and it means I don't get any useful grouping of POs since the 'project' unit loses its meaning.

I will add my voice to this. Currently, having to put the PO number as the invoice footer text. Much better to have a PO prompt per invoice, which if left blank, defaults to the project PO reference. Also worth noting, not all invoices are against projects, so having a prompt for the PO number makes sense to me.