Our group had gloves enchanted with effects like this, and they worked just like the spell. Take an action to activate, then next attack is at +20. For a bow like that, you could take one of two routes. First, there's a Bow of True Arrows in Sword and Fist, which casts it an unlimited number of times as a wand would.
Second, and more appropriate I think, make it part of the item's overall effect. Make the character take a full-round action including prepping the bow and firing to activate the ability; they can make a single shot at +20 three times a day. At 5th level, fighter-types are getting close to two attacks in a round, so in a single level it'll be a choice between two chances at striking or one sure thing. In the hands of a caster-type, it lets them take part in combat once in a while with some effectiveness beyond their spells.
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Oops, forgot about pricing. Standard +1 bow would indeed be 2,000 gp. Plus the 3/day true strike (use-activated), would be 1,200 gp... 1st level spell, at 1st level caster (1 * 1 * 2000 = 2000), 3/day (2000 * 3/5 = 1200). And the similar abilities thing applies mostly to Wonderous items. For weapons like this, you simply add the values together making the total 3,200 gp. Weapons like the dagger of venom were made exactly like this.