It so happens that the simple text was part of a group mailing, if that's relevant.
Bingo.
https://support.sasktel.com/app/answers/detail/a_id/16890/~/understanding-why-a-text-message-may-be-charged-as-a-picture-message
Some wireless devices may convert text messages (SMS) to picture messages (MMS) in certain cases, which can result in you paying the $0.75 pay-per-use Picture Messaging fee for each sent message. This only applies when you don't have unlimited picture/video messaging.
This can happen when you:
Send a text message longer than 160 characters (some devices may split the message into separate 160-character messages and send them as MMS messages; see Text message character limitations)
Have the "Send MMS" option turned on in your phone instead of SMS; or your phone is set to automatically send all messages as MMS by default
Reply to a picture message, even if your reply is text only
Include a subject line in the text message
Send the message to an email address
NOTE: If you're sending a group text message to several people, if even one of them is an email address instead of a phone number, ALL of the group texts could be billed as picture messages
To lower your messaging charges, make sure all of your text-only messages are sent as text messages (SMS). You may find this setting in the Text Messaging or Settings menu of your phone. See your phone’s manual for more instructions.
https://support.three.co.uk/SRVS/CGI-BIN/WEBISAPI.DLL?Command=New,Kb=Mobile,Ts=Mobile,T=Article,Case=obj(14694)
A text might turn into an MMS because:
one or more of the recipients is being emailed
the message is too long
the message has a subject line
there are pictures or attachments in the message.
On some phones, this simply happens because you are texting multiple people at once. Your phone will usually tell you when it’s converting your message into an MMS.
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