Until all portions of the agreed-upon price are in your bank account (or paypal account), the buyer has not paid for the snake.
All you have to do is cheerily state that you will contact him to make shipping arrangements as soon as the full price has cleared your bank account. To be friendy and chatty you can even add something like, "So just let me know when the second check is in the mail so I can keep an eye out for it!"
You just have to get better at saying "no" by saying "yes" to the situation you desire by restating it in a cheerful and clear way.
Something like this:
"Hi Matt,
Thanks for your correspondence. I'm afraid I can't ship until the total asking price has cleared my account. At the moment, my bank is still holding your first check, waiting for it to clear. Let me know when you send the second check so I can be sure to keep an eye out for it.
I'll let you know the moment the full price clears my account and I will be able to ship any M-W thereafter that works for you, weather permitting!
Best regards,
PJ"
You give too much information, and justify too many of your actions--it makes you seem less firm than you want to seem. He doesn't need to know when you deposited the check. The bank is holding it--the end. He doesn't need to know why it took you until it did to deposit the check. The fact that you are defending to him every tiny action you have taken with this transaction makes it much easier for him to push you. It is a "tell" that you can be pushed. Even if you contacted him to say "Just got your check! Thanks! I'll get it to the bank as soon as I can get out of my driveway after the blizzard passes!", that would sound less defensive than "not able to deposit it due to our blizzard conditions that are now just calming down but we still have high winds up to 40mph."
You don't have to be defensive, even, to say no. You don't even hardly have to say "no" to say no most of the time. You might have to, but you can take care of a lot of that by stating that you are "only able" to do something else. And you certainly don't have to defend it with detail. You also don't have to list things out--it sounds aggressive. You didn't really need to correct him about your gender (it doesn't matter for the transaction).
These are only my opinions, of course, but I'm pretty good at not getting pushed around by people while also not eliciting their irritation (or at least having them express it at me).
I bet Kathy is a master at that. Next time you are having trouble, post the emails and get people to help you phrase your response. It's just a skill that you can learn through a little practice and through having seen how other people do it!