Nothing says “We tested the edge cases” like offering me free regular shipping that arrives on June 12 for free or expedited shipping that costs $12.95 and also arrives on June 12.
True story: the state of Pennsylvania has shifted to a print-your-own-registration-forms format for car registration. (Your car must be registered for it to be legally drivable.)
When they used to mail your registration, they charged $2 for a second copy to be stuck in the envelope.
Now, they still offer the $2 fee for a second copy… but you’re downloading a PDF and printing it yourself. (You can print as many copies as you have printer toner and paper.) And if you do pay the $2, they give you the same PDF they gave you the first time.
So it’s really become a “are you paying attention?” tax on drivers.

Test your edge cases and your processes, folks.