"Last month, by a 4-3 vote, the Pennsylvania Supreme Court ordered the three-day extension, citing a 1980s precedent where a state court ordered a two-week suspension of an election when severe flooding made it impossible for voters to cast ballots. The state Supreme Court invoked its power under the Pennsylvania Constitution’s Free and Fair Election Clause to ensure that voters aren’t disenfranchised due to a public-health emergency it likened to a natural disaster."
"WASHINGTON—The Supreme Court on Monday refused to disturb a ruling by Pennsylvania’s highest court that extended the battleground state’s deadline for accepting mail-in ballots, a win for Democrats that gives voters more time to navigate postal delays and avoid in-person voting."
https://www.wsj.com/articles/suprem...r-mail-in-ballots-in-pennsylvania-11603149426
The PA Supreme Court and U. S. Supreme Court have both held the extension for receiving ballots is constitutional although the U. S. Supreme Court rarely rules on cases involving only state law and constitution.
The constitutional provision about legislatures selecting electors involves the electoral college process, not voting regulations and methods.