It’s possible you could argue that they are asking for an unreasonable late fee...but you would still be on the hook for 20% of $299 ~ $60
[3. An operator may impose a reasonable late fee for each month an occupant does not pay rent when due.
A late fee of twenty dollars or twenty percent of the monthly rental amount, whichever is greater, for each late rental payment shall be deemed reasonable, and shall not constitute a penalty.
An operator may set a late fee other than that permitted in subsection 4 of this section if such fee is reasonable. The operator shall have the burden of proof that a higher late fee is reasonable.]( https://www.lawserver.com/law/state/missouri/mo-laws/missouri_laws_415-417)
I’m also curious how much weight the phrase “each month” in (3) carries. A late payment fee is common, but I’ve never heard of a fee per each day late.