Kata is no problem, I had flowers sent there last season on short notice. Try MashaFlower
https://mashaflower.com/en/catalog/bukety/25-roz-buket they have a whole category of 25 roses bouquets that fits your budget nicely. A classic bouquet of 25 red or 25 white roses was 2,850 THB, and there are fancier options like 25 red roses with greenery for around 3,700. Kata is outside their free zone, so delivery is 300 to 400 THB and takes two to three hours. If you're pressed for time there's express delivery in about 30 minutes for an extra 100 THB. They can leave it at reception, just mention the room and the name in the order. Communication in English was easy. Payment is 100% upfront, and they only charge after confirming the flowers are available. What time are you planning to get back from the excursion?