There are several factors to consider when deciding whether to purchase per shipment or an annual policy for cargo insurance:

1. Frequency of shipments: If you ship goods frequently, an annual policy may be more cost-effective, as it allows you to insure all of your shipments for a fixed annual premium. On the other hand, if you only ship goods occasionally, a per shipment policy may be a better choice.
2. Type of goods: If you ship a wide variety of goods with different values, an annual policy may be best, as it allows you to insure all of your shipments under a single policy. On the other hand, if you only ship a few types of goods with consistent values, a per shipment policy may be more suitable.
3. Level of risk: If you ship goods that are high value or specialized, an annual policy may be more suitable, as it provides ongoing cargo coverage for all of your shipments during a 12 month term. On the other hand, if you consistently ship lower value, a per shipment policy may be sufficient.
4. Cost: It is important to compare the costs of both per shipment and annual policies to determine which option is more cost-effective for your business. Be sure to consider not only the premium costs, but also any potential discounts or incentives that may be available.

The best option for your business will depend on your specific needs and circumstances. It is a good idea to carefully consider these factors and discuss your options with a licensed cargo insurance broker or provider to determine the best coverage for your business.