Battery not keeping charge and taking a long time to charge

Have you tried a Hard Reset of the device, not sure of your model but this article should help, the section you want is towards the bottom.

How to Reset an iPhone (All Models)

When the unit restarts you will need to enter your password and that’s it - see if the phone now looses less charge in general use.

Another thing you can do is take a look at the Battery Usage section (my iPad’s is below).

When fully charged and you disconnect, if you don’t use your device expect Usage to be about 1 to 2 min per Standby 1Hrs. The area above “Time Since Last...” breaks things down a little so you might see wants using the most battery even when it’s just sitting around.

You don't say what model you have and it doesn't appear to be in your profile, that and your location will influence price - also I believe earlier models might be easier for third parties to undertake the task. You're best to call in to your local Apple Store and see what they say, you can then do a search in your local area for third parties/prices as a comparison.

Not withstanding the above, I believe some iPhone models had battery issues (6s regarding unexpected shutdowns) and as such Apple might be helping out customers with such devices, again it's something you need to enquire about - but even if the phone is not one with a known issue you still might have a troublesome battery that Apple I believe can test and if it falls outside of there limits may offer to replace free of charge.