She talks constantly about anything and everything. ... I have made comments about "really needing to get things done" or "being way behind" and have even had earphones in: she just keeps going.
After explaining it to her keep looking at her while you wait for the information to be absorbed. If you need to repeat what you said then so be it, she being such a nice person and all (you must keep it friendly if you work closely).
If it's clear that she can't understand what you're saying or ignores your protests and just keeps going then explain that you don't like to ignore her like she is doing to you but if that's the only solution she has to offer then you'll accept her idea. [This is your idea, let's do that.]
Our store owner knows it is an issue and has spoken to her a few times in the past but she is touchy about it and she really doesn't know she is doing it.
Ask the owner to give her work that can't involve yakking to you, like calling the customer base.
Ask for an additional duty that gets you out of the office, deliveries, picking up supplies, etc. then you get a certain break once in a while - that might be relieving. [Ask her to hold stuff while she's interrupting your work, EG: You're watering the plants, thrust the watering can into her hands, walk to the next plant, retrieve the waterer or get her to do the next one - remove the reward from interacting with you].
... often it's personal or just rehashing of work things. She likes to retell stories again and again.
Say you dislike knowing personal details. When something is a rehash remind her that she has explained that already and is taking away from the time needed to do what you were asked to do; was what you were told before unimportant ...
I work alone in the office with her everyday and do not want it to be unpleasant. Plus, besides this, she is a very nice person.
Yes, but is she polite or needy, nice or annoying, bearable or overbearing, Politeness Man (a polite nuisance)?