This story is hard to follow because it got renamed after Blackmailed 2. It was originally called Blackmailed but got rereleased as Friendly Trade and parts 2-4 have the sub title Payback 1-3 just to keep things confusing.

The correct reading order is

Blackmailed 1/Friendly Trade 1
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Blackmailed 2/Friendly Trade 2 - Payback 1
/g/785471/84b8c44fdd/ (This Comic)

Friendly Trade 2 - Payback 2-3
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After Friendly Trade finished two more stories come out that directly follow this story line. In order they are

Harsh Judgement 1-3
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Community Service 1-3
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Base +6