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Guest post are free, while the paid links require payment .

Basically if you pay for guest post it will be counted as paid links. And google suggest using rel="sponsored" for this kind of links.

In reality we pay bloggers or webmasters for publishing guest post , which is not recommended by google.

You can use tools like Postifluence or Upwork to find freelancers to do it.

However, don't go for the traditional Guest post site list. Do the manual work, use recourses to find fresh and Low OBL sites.