In the Core Rule Book, it says
You can carry an amount of Bulk equal to 5 plus your Strength modifier without penalty; if you carry more, you gain the encumbered condition. You can’t hold or carry more Bulk than 10 plus your Strength modifier.
What does without penalty? How I read it is you don't count negative strength modifier but I think I could be reading it wrong.
"Without penalty" meaning the character is not suffering any conditions or penalties from carrying up to 5 plus their strength modifier. Negative strength mods still matter.
Ok, so I was reading it wrong good to know.