Re: Would it be constitutional?
I don't know what the law says about this, but why not ? A lot of laws exist to explicitly impose or nullify contractual terms. For example, in the UK, the UTCCRs (Unfair Terms in Consumer Contract Regulations) basically say that if a consumer contract contains certain types of clauses then those clauses are automatically void and unenforceable. Similarly, the SOGAS (Sale of Goods and Services) Act basically says that any consumer contract is assumed by law to contain certain clauses giving consumers certain rights.
As to inter-state trade, that's already "complicated" by different tax laws, different age of consent laws, etc, etc, etc.