Allow conditions (similar to auctioning and vendoring) for posting operations.
For example [if destroy > dbmarket send to DE-alt].
I would use this to send transmog items for DE, herb for milling, ore for prospecting, but ONLY if the destroy value was higher than the value of the item itself. If the item is higher value I would send to my bank/AH alt instead.
Came here to say this.
A temporary / simpler solution would be to just allow custom strings in the mailing > "keep this amount" field. This would allow users to create their own logic within the custom string, without the need for an interface update. Currently custom strings are allowed in auctioning > post cap, etc. so I don't immediately see why they aren't allowed the "quantities" for mailing (and warehousing) operations.