Invoicing for parts

Hello there, as a freelancer, I feel we get stuck between a rock and a hard place in terms of invoicing.

My clients will often have just one piece of the work done, then will disappear. As PPH do sometimes give refunds unexpectedly (it does say that you have notified me that it may happen - but I don't ever seem to receive this notification). I am forced into a situation where I have to bill the client as I've done the work, they have gone missing and I'm concerned that you're going to refund them. Even if they haven't given me feedback about the work and it may require changes, I'm still forced into this position to protect my own livelihood. 

If a client has bought two pieces of work as a package, but only one piece has been done, I am being forced to bill them in entirety as you don't have an option to either change the amount I'm invoicing or to add a discount to it.. 

Please can you amend this... an invoice either needs to be able to be changed so that there is some money left in escrow that wasn't used.. or I need to be able to give a discount so that that same money can get left in escrow. 

I know you guys are trying your best, and are continually looking to improve.. also the search still doesn't appear to be working properly... a client has terrible difficulty trying to get results that make sense and are relevant.. will you be updating us on what you can do about this? It's been a very long time that you've been working on it but I haven't seen any improvement as yet... 

thanks