Signature Paid Mods Question

[Sim9]

Apoligies in advance for some potentially obvious questions, please don't flame me if I missed something

I looked around and couldn't find the signature rules (I remember there being a thread somewhere as that's what I used to set the size of my current signature. Closest thing I found was this thread from 2004).

I was trying to find them to see if links to paid modifications in signatures are allowed or ruled out. Thanks!