You are technically supposed to have a permit to import any plant material (though you can import up to 12 live plants with just a phytosanitary certificate.) There is a special permit just for seeds that is free, but you may have difficulty convincing the seller to use it as it is a bit of extra hassle and I don't think customs actually cares about seed shipments. From the seller's point of view it's extra work for no benefit since the seeds would almost certainly arrive safely without it.