"けっこう(結構)" has many meanings in dictionary , but not all of them are used frequently in daily life.
There are 3 most frequent meanings , which are 1) "nice, wonderful, sweet", 2)" good enough, fine" ,and 3) "quite, very".
As you understand, " I do shopping and a cashier asks if I need receipt or not I can answer using けっこう". Here, けっこう means "good enough". You can understand it as " I am good enough without a receipt."
There is similar express in English↓↓↓↓↓↓.
A: "Would you like to try some of my new recipe? " 新しいメニューを試してみますか？
B: "No, I'm good" 結構です。
Hope this helps:)