Please help me solve the delivery issue in e-commerce!

Hello to all non-coders!

Help me with advice on solving my issue. I am currently working on an e-commerce project.
The delivery settings are set to flat rate. The system for configuring these data is quite simple. However, when adding a product to the cart, the delivery cost is not taken into account. Sometimes it appears, sometimes not, sometimes on the desktop version of the site, but not on mobile. It also sometimes appears on the checkout page. I don’t know how to solve this problem; I’ve already written to support.

This is very important for my client; he cannot launch his online store without it. I will be grateful for any information.
Thank you!


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