Hi there,
I read your project description and I think the errors you are receiving are both from the client's end.
Since you are most likely using PayPal as a payment gateway, it is possible for PayPal to reject or decline some cards, especially if your customers are mainly from Nigeria. This of course will mean that you'll loose most customers since majority of the Nigerian users do not have a PayPal account or simply do not pass PayPal's multiple card verification checks even if the users are legit.
However, I can help fix this by integrating a new payment gateway to your Shopify store to allow for local card payments which most Nigerians are quite familiar with. What this means is that in addition to your current PayPal payment gateway, I'll add another local payment card option.
This will reduce the checkout process and provide a more user friendly method for your customers to make payments on your store.
Although your budget minimum is $250, I can deliver this project for $200 in 3 days if hired.
If you have other questions, please reach out directly via freelancer live chat.
Cheers.