There're three type of discount you can set up:
If your online order is every order will get 20% discount with minimum spend, please select Percentage of entire order (%)
If your online order is every order will get $10 deducted with minimum spend, please select Fixed amount
If your online order is get $10 deducted for every $100 spend with minimum spend, please select Get $X for every $Y spent
Here's the steps for your references:
Login Backhouse > Online Ordering > Promotion settings
Click Add Promotion > Select Price Discount
Set up the promotion details (Kindly refer below directions)
Hit Save to done the settings
If you select Percentage of entire order (%), kindly fill in :
Discount percentage
Minimum order amount
If you select Fixed amount, kindly fill in :
Discount amount
Minimum order amount
If you select Get $X for every $Y spent, kindly fill in :
Discount amount ($X)
Order amount ($Y)