I can get back after using this for 1 month but here is my estimation:

Free Tier covers the requests and execution time for small Lambda functions.

If execution time is within the Free Tier, Lambda costs may be negligible.

Amazon S3:

Assuming <1 GB of storage and minimal requests, S3 costs should also be very low, typically within a few cents per month.

Amazon EventBridge:

EventBridge is very inexpensive, especially at low volume (under $1 per month for your triggers).

Twilio WhatsApp Messaging:

Twilio charges around $0.005 per message.

180 messages x $0.005 = $0.90 per month