Recently, Will Schmitt of Classy posted a blog discussing the limitations of text to give and text to donate services (he distinguishes the two). The blog does point out some interesting considerations, but it overstates the limitations of text to give.
Whether linking a card or a bank account, if the donor only wants to enter credit card/account details once, she will have to create a PayPal or GMG account to store the account details to use in subsequent donations.
We have created custom PayPal implementations in the past, but we don't really do it anymore. Basically a programmer has to create a web-based form that has the options, and then pass that data to the PayPal checkout form.
Take a look at our Frequently Asked Questions page. It's a great place to start if you have questions. In the meantime, here are some common questions we get from customers.