Is the phrase 'wonderfully easier' grammatical?
"easy" is an absolute term, while "easier" is a comparative.
Shopping at most convenience stores is easy (that's why they're "convenient"), so the advertisement is claiming that shopping at their store is more easy than shopping at the competitors.
Adding the "wonderfully" adverb is hyperbole intended to intensify this. Taken literally, it means that it's wonderful how much easier it is.
There's nothing ungrammatical about this. "Easier" is an adjective, and you can generally use adverbs like "wonderfully" to modify an adjective.