So you've got some experienced keepers giving solid experienced advice - your in a good starting place, I've been keeping boxies for, gosh more than 30 years, one thing I've learned is that their captive care has come a LONG way & options to improve their lives and enable them to live longer have grown as well.
Here is a link to a good starter caresheet - general to EBT's not just the 3 toed
http://www.austinsturtlepage.com/Care/cs-easternboxie.htm if you scroll around ATP - that site you will find a forum dedicated entirely to turtles and their care, while forums are good for the give and take response one of the best things about that site is their archive of care articles - tons of good info. - I don't want to overload you overmuch esp. as you 've got good help and good plans, but a few tips that I think would matter most right now -
most of the box turtle care books haven't caught up to the best care info so ATP & turtleforum is a good place to go for up to date diet options and captive care info - I don't recommend turtle times, just because they haven't really caught up and don't get as much traffic or seem to have as many long term breeders, ATP/turtleforum mirrors this one and is also a good place to check classifieds for additional turts, that being said - turtles don't get lonely and more than one is more than twice the work - fighting biting, sharing illness - it's all a potential issue, plus you would want to specifically determine the type of boxie you already have to avoid integrade breeing issues, she/he seems like a slightly atypical 3 toed more like a
carolina carolina than a
triungis skull shape and coloring both on the skin and the shell (& that is not due to the bit of prymiding). Your turt actually appears to be an integrade itself, a plastron shot at a slight front to back angle, a straight on tail shot and a side view would make sexing almost 100% accurate online if the pics are good.
Any live food typically found in the wild is a good add on, earthworms - many people use baitworms - naightcrawlers are a favorite, a few waxworms are good - avoid mealies too much chiton crickets are good if you can contain them or don't mine pulling off the jumping legs proi to feeding, my EBT's love rolly polies, slugs, assorted beetles (june bugs are eay to get in season), and the domestically bred feeder roaches... anyway live food stimulates appetite and mixed with vegies is a good diet if you check the link info I posted there are numerous feeding schedules based on the diet make-up and proportions based on turt size.
If you do live foods & veggies & can supplement with a good turtle food - I use small amounts of moistened zoo med aqautic turtle pellets that is a great diet staple, many swear by jurassidiet for turts, I do see an improvement when corn & soy fillers are avoided - the only dog food I ever offer is for undernourised turts (non-eaters) & rescues that need the more intensified calories & then I use simple ingredient dog foods, I like wellness brand 2 protien mixes- just a meat, a veggie and a rice formula no corn/soy etc which can block nutrient absorbsotion in a turt.
Once yours is outside D3/calcium not so much of an issue but even with UVB supplements are a good thing a good quality herp dust like repcal calcium D3 & herptivite are my first choice.
FYI - turtlesale is the most ill reputed turtle place on the web - their death rate is apalling, they mis label turtles often and their practices are 'iffy' at best, far better to check the kingsnake classifieds...remember too the minumum quarenteen before putting 2 turts together is 90 days, six months recommended & if you hibernate, a full year, russ gurly's book on NA American turtles is a good resource, and turtlesource.com in florida is building a rep with some care info & animals for sale (rep. = pricey but quite good). Good luck with your boxie and good for you in devoting yourself to it's care.:thm: