If you go to the black market and then to the my purchases area you can enter the code GetInline it will give you 150 sardines. You can then go into options on the second page and reset you progress you will still have the sardines but you can enter the code again. You can infinitely repeat this. To get every item in the black market and the bonus shop maxed you will need about 1400 sardines, it takes a bit of time but its worth it. you can beat 4000k story mode in three days.

UPDATE 6/16/2022:
based on comments; Windows savegame location is here:
<Steam-folder> \userdata\ <user-id> \589870\remote
savedata.dat - save file

Backup the file and proceed with guide. Restore savegame.dat to keep old saves but with extra sardines. If you wish to start from scratch and earn your meal honestly then delete everything in \remote folder while the game is running or in offline mode, else Steam will re-create \remote.

I have also been told by other people that 1400 is too much, and some have said it is too little, so run your own calculations, including money required for the bp shop.

This Guide is still working as of 6/16/2022 and I imagine will never be patched.

If you need help with this guide feel free to message me.

Thank you for all the support on this guide, and please for the love of god stop posting doge on this guide, its not funny.