To enable price rounding, check:
Price Rounding
You can then choose the rounding rule you want.
Price rounding applies only to discounted prices, meaning when the price shown on the offer is lower than the original Shopify price.
It does not apply to the Default price.
If rounding doesn’t seem to work, make sure the offer actually has a discount (that the offer price is not set to Default), or contact MoonBundles support.