Hi, Please excuse my poor English Skills i am German...
if i Buy the Bonus Slot item i dont get any more places to store these Items...

are you sure you bought it? the price should have changed to 60k
if that's the case, the gui just didn't update.
maybe try buying a health potion and see if that updates it...
for further testing you can use this command to cheat gold:
/c remote.call("rpg-items","add_gold", game.player.force, 60000)

I also have that problem as well. When I click on Bonus Slot, nothing shows up below like the rest of the items.

cant reproduce... please send a savegame..

Where do I send the file? Also, when I change prices, the bonus slot remains the same price too

if you don't want to upload it on a public hoster, you can send me a message on the forum and attach it there
https://forums.factorio.com/memberlist.php?mode=viewprofile&u=50852

http://www.mediafire.com/file/vk0cf6zkdavapgr/Bonus_Slot.zip/file

couldn't reproduce it with your savegame and mods either.
i used the command i posted to cheat some money and bought the bonus slot in the market. it updated immediately and i was able to buy 5 items.
the "+1 inventory slot" in the talents window only adds 1 normal inventory slot where you can store normal items like ore, plates, assemblers etc.

I'm using version Factorio 0.18.8 with RPG Items 0.18.8. Idk if that changes anything. Do I have to have a full inventory to buy it?

no but you need enough money, it costs 50000

I have tried removing all mods so RPG Items is the only one, but it seems to still have the problem of being unable to buy. I used the gold cheat and filled all four slots with items and clicked on Bonus slot, but no lower menu popped up like the other items. Also, how do you toggle aura from flame cloak.

you can toggle the aura with a left click
the price of the bonus slot is hardcoded, you can't change it with the price multiplier
just to make sure:
- you used the savegame you sent me
- you entered this command: /c remote.call("rpg-items","add_gold", game.player.force, 50000)
- the gold amount under your items shows a higher number than the price of the bonus slot (>50000)
- you made a right click in the market on the bonus slot
- nothing changed
?

Oooookay. I got it. I was suppose to "Right-Click" the icon and not left click like everything else. Thank you very much. :)

i think you can't buy anything with left click.. it just expands the combination tree