And why would you bring through a single cart? 6 seconds and a 10 foot radius should be able to get you multiple cartloads. And you should be able to haul something worthwhile back again - even if it's just sand.
Those ships you chartered cost 10,000-30,000gp each plus the price of the ports to support them and training their crew. Spending 18,000 to get instant secure travel seems like a bargain.
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