Apple themselves offer a battery replacement service for a fee, and it seems to be a reasonable fixed price (where I live). You might still be able to get them

Apple themselves offer a battery replacement service for a fee, and it seems to be a reasonable fixed price (where I live). You might still be able to get them to upgrade that Macbook's battery. Don't use an Apple Reseller for this without checking Apple's battery replacement price first. The reseller I went to quoted 2.5x the price, and said they were unaware of Apple's price (seems unlikely), which they could not match.