Why do you say store bought is pre-steeped?
If it's anything like the local B&M here, you can tell it's aged/steeped because of the color. If you've ever create a DIY blend, it may be one color one day and another a week later, whether it's deeper/darker or another color altogether. Most small-time B&M's don't want to have to explain aging/steeping to every customer than walks in the door as some are relatively clueless, will forget and simply vape away thinking the flavor is horrible and never come back. Not a bad thing, everyone has to start somewhere and that was me years ago

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That said, if you tell someone they really need to wait a week to use the juice they just bought at your store, they most likely aren't going to buy because they want to get started now, not next week. If a B&M ages/steeps ahead of time and rotates stock as they should, then it's straight to vape.
Fresh Mixed get's shipped when you order online, so the first 3-5 days are already out of the way.