Heya, thanks for reaching out. A few ideas for you:

– there are sophisticated international batch SMS solutions that also offer other marketing tools like email and automated calling etc. Check out their pricing. Message Bird and Twilio pop to Mind. https://www.messagebird.com/en/

– have you investigated a subscription model? Get them to sign up and pay a monthly fee and you auto deliver? Would be interested to know if you customers would be keen on that. You could ask them in your next txt.

Hope that helps. Happy to have a chat. Book here https://calendly.com/rachel-adams-manaaki/15minonline