That's a good excuse, as far as excuses go. It's not like they didn't know this in advance. Even before the 'rona, online shopping has been on the increase every year.
Companies have been adapting to accommodate their customer base for years, bringing on additional staffing for the holiday shipping rush. All except for USPS anyway.
For years the USPS has complained that they need more of our tax dollars. They figure that it is in their interest to be less than efficient in some cases to help promote their welfare mentality.
For better or worse, I buy a ridiculous ton of stuff from Amazon (hundreds of orders per year), and as soon as Amazon decided to use UPS for bulk and USPS for final mile, delivery timing and accuracy decreased by at least 30% and this change happened years ago at this point. USPS has had forever to get their stuff together and they simply don't care.
After the first few years of that Amazon seemed to back off of it and now they're using their own delivery service, which is actually quite good most days.
FWIW, the only thing worse than USPS is FedEx. FedEx just absolutely destroys packages and takes forever to get them to me.