The ADA, unfortunately, does \*not\* have the teeth that the other equal rights measures do. The definitions of disability have changed over the years to where the gentleman who was instrumental in writing it and getting it pass is no longer covered by it (has epilepsy controlled by meds). They also really messed with what is considered a "burden" to the defendant (employer/store owner/etc).
BUT this sure seems like a clear case, where they could have assigned one of the clerks to be on the alert for disabled customers needing help checking out and to help them. It's not ideal but it is a way they could follow the law and they didn't even do that.
OP here's a guide to filing a complaint with your local office it includes what info you need:
$1 Here is where you can file online
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