If you have already confirmed there is sufficient funds in your account but your creditcard is being declined when you try to upgrade your account, it may be necessary to inform your issuing bank of the following.
1: Changes in spending patterns:
For fraud prevention purposes, some banks refuse payments for transactions that do not fit your normal spending patterns.
If it is unusual for you to be making online payments for services such as SupaDupa, you may want to advice your bank of this.
2: Payments to companies in a different/foreign country
SupaDupa is a United Kingdom (UK) registered company.
As with point 1 above, if you do not live in the UK and it is unusual for you to be making payments to companies outside your home country, your bank may be refusing payments until you advice them otherwise.
You may need to inform your bank of the above information and authorise them to allow payments to SupaDupa ME Ltd.
Alternatively, if that still does not resolve it, try again using a different creditcard.