+(Test this)
+one-time/per-customer/? changes in rates and descriptions ('remembered
+invoices'): implement by creating a new package on the fly... but it isn't
+associated with any agent types so it won't show up for other customers to buy.
+(but also... make sure they go away when the customer does! - need this? :
+ one-off package edits! : need a cust_pkgs or cust_part_pkgs or something table,
+ with custnum and partpkg (like type_pkgs)
+(what happens if you hit "custom pricing" but the pricing is already custom?)
+