Why does Vinted shipping cost change so much?

A few months ago I was surprised to see that shipping went down to ≈1,39€ within my country, but then it went back up to ≈2,3€. These past weeks it changed to 0,99€ WHICH MADE ME SO HAPPY, but again, now it’s 2,3€. I feel like I’m in a toxic relationship. Is it the same in your countries?