Fix Big Commerce Shipping Issues

I'm experiencing multiple shipping-related problems on my Big Commerce store that need urgent resolution. The issues include:

- Incorrect Shipping Rates: Rates during checkout are too high in some cases and inconsistent.
- Carrier Integration Problems: Some locations cannot be shipped to at all.
- Checkout Process Errors: Overall, the checkout process is error-prone due to the above issues.

Ideal skills and experience for this job:
- Proven expertise in Big Commerce
- Strong understanding of shipping integrations and rate calculations
- Experience troubleshooting checkout process errors
- Attention to detail and problem-solving skills

Please provide examples of similar work done. Looking forward to your bids!