Once contractually terminated, THAT’S IT! (No take backs.) The surrogate should then have every legal option available to her as would be afforded a natural pregnancy, including, finding alternative adoptive parents for the child, adopting the child herself, or terminating the pregnancy where laws and situations permit.
In the scenario of accepting 2 babies of a triple-pregnancy, each baby would require a separate “termination of contract” and in such case, the bio-parents should not get to “pick and choose”. First two babies out fulfill the remaining active contract. Third baby still pays out the $5k termination but is either surrendered to the state, immediately adopted by the surrogate, or an adoption arranged by the surrogate.
I know it’s quite a bit of “ick” to be so cold and calculating, but that’s the purpose of a contract. Each reasonably plausible scenario should be considered by both parties and if they can’t live with one of the scenarios coming to fruition, then they shouldn’t enter into such a scenario rife with ethical quagmires.
Ethics Alarms 
Still gaining something when breaking a contract isn’t an equitable outcome.
If the birth mother terminates the baby for elective reasons, she should be on the hook to replay. No way should she profit from changing her mind.
I agree with you on that – but that’s not in the comment I wrote, is it?
Does surrogate motherhood ethically fall in the same category as selling a baby or selling a kidney? Certain practices are so rife with pre-unethical conditions that these practices are unethical, and given this outlawing these practices may be the wiser solution. Parenthood is a gift that is not given to everybody, and painfully as it sounds but people should accept that you cannot always have what you want.
A better way would be to make the adoption process easier, with more legal certainty for adoptive parents.