Afraid ebay is very rigid on dispatch dates, on occasions I have explained to buyer the problem and either arranged to cancel and resell or if only a matter of a few days just marked as dispatched.
 
This does not help once ebay have recorded a late dispatch, but might help in the future.
 
This can be very annoying if buyer is buying several items over a period of several days, unless seller can ask them not to pay until invoiced. For private sellers with ebay’s inflexible  Simple Delivery even worse.