Paulette, when you say CUSTOM, does that mean you are creating the item at time of purchase? If so I follow these guidelines.
I get quite a few buyers that write to me ( for example ) on the 22nd and say I need these by "the 28th", will they arrive on time! If I have to create them , and do not have any in stock then my hands are tied until they pay! Cutting Fabric or handstitched work limits you to reselling it if the buyer doesn't show.
In cases like these, I post it, convo them that their RESERVED or CUSTOM listing is up and that it is valid for 72 hours. If they don't pay within 48 hours I convo again and tell them it will be up for another 24 hours. I have to make sure they realize that the date of shipping depends upon the date of payment, not the date they wrote and requested the item.
I have only had to do that once or twice, most buyers that have chosen a custom made item pay fairly quickly for me.
If by Custom, you are referring to RESERVED FOR, ( meaning you have it in stock and it is ready to ship) then I do a similar listing, but use those words vs Custom, and only keep it up for 48-72 hours and ALWAYS attempt at least twice through Etsy Convo or personal email to contact the buyer. I never leave it up past 72 hours.