Most of our merchant partners offer open denominations, but some (e.g. Hudson's Bay) only offer fixed denominations ($10, $25, $50, $100, $250, $500). The sender is notified when choosing a denomination which merchants are not available as redemption options.
The merchants showing for your gift are the ones that are available to choose from for your gift denomination. The tech team is working on a feature that will allow you to split your gift into smaller denominations to redeem at multiple merchants. It should be ready in the next month if you prefer to wait.