There are existing laws prohibiting tampering with elections and with postal mail, but those do not stop people from committing crimes (as you highlighted). Thankfully, the elections process of in-person voting, verifying the identity of the individual casting the ballot, and certifying the ballot count has ensured the tampering of ballots to be a non-issue in past elections.

The large growth of mail-in ballots (especially if made the default method for all citizen) expands the risk of ballot tampering to the postal system and individual homes. In theory, the risk of ballots being tampered with increases as more people are handling and have access to the documents. Doing so at a large scale is unrealistic (IMO), as I stated. Again, this is most likely why Republicans have largely agreed to accept this method of voting so long as it can be assured that the person filling out the ballot is who they claim to be.