Gaining reputation to reach 100% to unlock special merchandises (weapon attachments) can be a tedious process, so I added one more method to increase faction reputation: by donating demanded items to the main trader.

To do this, first talk to the main trader (such as Sidor and Filip, basically people who give you serum orders) about "Demands". The type of items they list out will be the ones you can donate. For example, if he says he needs serums, then when you trade with him, you can put serums up for sale; but instead of pressing the Trade button, you can press the "Donate" button right under it. The item will be donated to the faction in exchange for some reputation gain. The amount to gain is related to the sell price of the item.

Note that the "demands" information also dictates how much the item costs when you sell to them. Items that are demanded by the trader will sell for a better price for you.