Spell in a can

The fly spell can only be cast by Wizards or Sorcerers (or Clerics with the Travel domain, but how many of those do you know?). It's also a 3rd level spell, which means your Wizard would have to sacrifice one of his daily fireballs to prepare it, or your Sorcerer would have to choose it as one of only 4 3rd-level spells he can ever learn. (Of course, your Cleric would be limited to casting it once per day, or memorising it as a higher-level domain spell). There is also the Air Walk spell, which is Cleric/Druid level 4. However, that's one less Cure Critical Wounds for the day.

Winged boots cost 16,000 gold pieces. Other means of magical flight are similarly expensive. And the availability of cohorts and mounts that can fly is fairly low in most games.

Given that a huge portion of the effectiveness of the average Fighter relies on his ability to get up close to the enemy, if that enemy can fly, the Fighter really needs that ability also. Suddenly, that potion of fly starts to look really useful, doesn't it? Of course, at 750 gp for a mere 5 minutes of flight, it's quite pricey, but most groups should probably invest in one or two. (Not that you're about to be facing flying creatures, of course...)

Likewise, spells like Resist Energy and Protection from Energy are very useful, but they are also in short supply. The former is 2rd level for most casters, while the latter is 3nd. Again, potions of these spells are available, and should be considered.

It's funny - the current group don't tend to use disposable magical items, except for healing devices. Which is fair enough - disposable items are a drain on resources that are quite limited, and in the long run permanent items are a better investment. However, there are big boons to be found in the careful use of such items.

Also, it's worth noting that potions of healing are probably a waste of funds at this point. Using such a potion requires giving up on a round of attacks to regain, at best, 3d8+5 hit points. Far better to rely on the Cleric for emergency healing, and using wands for healing the group between combats.