The soonest I can get a humidifier for his terrarium is in a couple of weeks. Will my ball python be okay with low humidity(20-30%)?
Yes. You can make humidity by just spraying a little bit of water in the tank each day. Put some moss in the tank and redamping it each day with warm water. Until you can get a humidifier, do that.
I I suggest using under the tank heat (UTH) so your snake can warm his belly and digest his food. You can use a heat lamp, but you are almost guaranteed to have humidity problems which is a direct cause of a bad shed (a shed that comes off in pieces or leaves pieces behind). There are all sorts of under the tank heaters, the most common UTH is a reptile heat pad that can easily be found at a pet store. You want the tub/tank where your snake lays to be about 88-92 degrees. This should be the temp that the tub floor is, not the temp of your heat tape/UTH. My heat tape is set at about 96 which is a tad hot for a snake, but you lose some heat because the tubs have air gaps. It