I have about 5 years experience in logistics. Yes, it is illegal to order seeds online, and have them shipped to Texas.

Scenario 1: If you order seeds online to have them shipped to Texas, the online store might notice your shipping address, either automatically via software, or during order filling, and just cancel your order.

Scenario 2: Your order is filled and shipped. Federal LE monitors the records of these online retailers. There is always a chance they will audit these records and find you have obtained seeds. They might never go after you, but the online retailer instead. Or they will go after you because you are simply low-hanging fruit and an easy arrest. They don't care who you are or what they have to go through to get you, you are an extra arrest to further their careers.

Scenario 3: The US Postal Inspector, fully aware of what is going on, intercepts and opens your parcel and notifies the authorities. The Postal Inspector does not need a warrant to open your mail. If They ship through a private carrier, that is a different story.

Scenario 4: The online retailer is actively monitored and your shipment is immediately intercepted. They will either seize your order, or allow it to go through and surveil you.

I am aware of all four of these cases happening. I happen to know that the most likely case is they fight this type of victimless crime bureaucratically. They will periodically collect the order records of these online retailers and audit them. If they find an order has been shipped to an illegal state, they will 1) go after the business and/or 2) create a database of illegal shipments to conduct further investigations. When they come after you, it won't be for months or over a years later because the bureaucracy is so slow to function.

It's basically a crapshoot. But times are changing now and Texas is about to allow growers for MMJ inside the State, so the heat may have died down. But it's the government. They will spare no expense to waste money.