In an open letter to heads of Walgreens and CVS, Republican attorneys general from 20 states warned that shipping abortion pills within states is against the law. Both companies earlier this month said they plan to ship the pills pending FDA approval. 
The letter claimed that federal law "expressly prohibits using the mail to send or receive any drug that will “be used or applied for producing abortion.”
"Although many people are unfamiliar with this statute because it has not been amended in a few decades, the text could not be clearer: 'every article or thing designed, adapted, or intended for producing abortion … shall not be conveyed in the mails,' the letter read.  
The Biden administration made the opposite claim in December. 
The law in question "does not prohibit the mailing of certain drugs that can be used to perform abortions where the sender lacks the intent that the recipient of the drugs will use them unlawfully," read a note from the Justice Department's Office of Legal Counsel. 
"Because there are manifold ways in which recipients in every state may lawfully use such drugs, including to produce an abortion, the mere mailing of such drugs to a particular jurisdiction is an insufficient basis for concluding that the sender intends them to be used unlawfully."
In plain language, the administration claims that the law can't prohibit the mailing of abortion pills, because the drugs have other uses that may not be illegal. 
"We reject the Biden administration’s bizarre interpretation, and we expect courts will as well," said the letter. "Courts do not lightly ignore the plain text of statutes."
Whatever the legal interpretation, this kind of pushback from attorneys general usually signals that a more formal legal challenge is coming.