I see, yeah 14Ah for $650 is pricey. Definitely could find one for less. The UPP batteries will be a bout half that for comparable amp-hours. Just make sure you get something with THE SAME VOLTAGE but same or greater Ah (amp-hours). BtrPower is also good from what i hear. The amp-hours is the capacity, what you're paying for, more amp-hours = more range. There's an 18Ah 48V BtrPower battery for $269 there on amazon. Also bare-bones, no case, just sort of a plastic wrap. You can put it in water-proof pannier wrapped in bubble-wrap. As long as it's padded and waterproof you're ok. These will all come with XT60 connectors which are pretty universal, but you can adapt with a crimping tool if you have something different on your bafang controller.
Also the amp-rating on a battery is theoretically a spec to get right but only for higher-amp systems. The rating on the battery that says, for instance, "50A bms" means it can output 50 amps before the current outflow is limited, it's like a circuit breaker in a house, sorta, but not really. In any case, your Rad power probably has a 750 watt motor, (750 watts divided by 48 volts equals 15.6 amps) which means you'll be fine with any battery that is 20 amp bms or greater. Most of the UPP and BtrPower batteries have 30A or 50A bms so you're fine no matter what basically.
One thing worth considering is just taking the battery to a real old-school hacker nerdy bike shop if there is one in your town. Try finding somewhere with hardly much of a sign out front but in the back of the shop it looks like a nuclear submarine scrap yard. Rad power is a very new and very retail-oriented company, like get them manufactured and get them sold, and after that we don't know you. I have had frustrating conversations with Rad power sales reps who don't appear to know the answers to real basic questions. A real ebikes nerd will have diagnostics tools... it could be one or two bad cells, could be the bms, could be just the connector... Even if it's a matter of replacing all or many of the cells, or replacing the BMS board entirely, it could be worth it, considering your case fits the bike. And they will offer you better advise than i can if they are looking at the bike.