Basically, the courier company that MC used was also running an illegal drug business by hiding drugs inside coffee bean sacks. Seme gave MC a bunch of paychecks 'for the coffee beans' to test him out (because if MC had accepted, that would've meant that MC was involved with the drug trafficking). It was logical to suspect MC since running a cafe in the country side really does seem like the perfect front for a drug business. MC doesn't know what's going on but he quickly realizes that something's suspicious and refuses the money. Seme thinks MC is smart. And the rest... pretty self explanatory.
oohhhh....thanks for explaining ٩(๑❛ᴗ❛๑)۶
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