Lightspeed was supposed to reimburse me for shipping, but I have yet to
receive a check since my problems began in March. I suspect a terse call
to their customer support would produce the desired response, but again
this requires more of my time, which is also accumulating.

Peter, don't use your own money to return their product.
When you call to obtain a Return Authorization, ask for their FedEx or
UPS account number to charge the shipping to.
Or, you can ask them to send you a prepaid return package.