I have set my additional words extra such that once the customer’s required word count exceeds the word count of a package, it will be cheaper for the buyer to select the higher package than to select the lower package with the exceeded word count. That way, they don’t select the lower package with the faster delivery time and request for tons of work.
I have also set the delivery time of my cheapest gig package such that I have plenty of time even in case that happens. 5 days for base package (5k words) which, in fact, I can probably complete in 1 day - - 10 days for intermediate (25k words) - - 14 days for top package (50k words).
The way in which the extras are set (and priced) decides whether or not they (sellers) will frequently run into issues with their customers.