One of the basic design principles is font use. Use at most two different fonts and use sans-serif fonts in body text. Fonts set the mood and chracter of the page. I also believe the the use of font weight can/needs to be used in a way to convey information, what information is helpful, and where to guide the reader/viewer.

Probably most apparent in pages displaying subscription plan differences, with the middle plan being the most attractive with being center, bigger, and often having a different color than its surrounding options. The combination of these and possibly even the reverse of scarcity ("most popular") can be very powerful in affecting user behavior.

This seemed very counterintuitive to me at first, but after some brief research it seems to make sense, as there's a balance between complexity for the user, complexity for the developers, and functionality provided.

Something that's seen all over ecommerce with statements like, only X left in stock, in X people's carts, sale for X more days.

Maximum of 3 days late for any credit.