The best way to earn experience is to go to the areas near the signpost at the far left side of the village and fight far more enemies than in quests but you'll also gain on much more experience points and coins, trade items, event tokens and upgrade items for your potions.

Delivery quests are for acquiring Postknight tokens which you can use to boost your base coin earnings, experience earnings, and quest reload time permanently.

Full five star upgrade at the blacksmith will unlock another bonus for a different statistic ie a 10% attack boost or a 5% critical upgrade.

If you unlock all four equipment items in a single set you'll get another boost.

Always level up your strength and vitality to boost your attack power and your health points.

Intelligence boost your magic defense and the experience that you earn in battle which in turn helps you to gain levels faster.

Agility upgrade increase your critical hit rate and your dodge rate.

Use the gems you have to recover immediately, or purchase new ones or watch an advertisement video for a free full recovery.

The event tokens are for the Blossom Festival that appears occasionally where you can buy event-specific goods.