I think even consumers are getting used to the fact that ordering online internationally is done mostly via credit cards.
Another question is, how would you handle automatic recurring payments?
To let costumes pay via wire transfer just give them your IBAN and let them pay. Or you take their iban and set up direct debit.
How to handle recurring payments? Well let your costumes set up recurring wire transfers with their bank (usually done here for paying rent or pocket money) or you just do recurring direct debit (usually done here for magazine subscriptions or insurance)
Most people just won't order because they would need to get a credit card.
(I agree with you that CC's are the only practical way nonetheless.)
That's why I carefully added "internationally and instantly" between brackets here.
There's also the Google Apps Marketplace to consider if your app targets the enterprise, which also hooks into the same payments APIs.
The bank is taking a considerable risk: whatever the merchant's expected transaction volume is, the bank could be on the hook for up to 6 months worth if that business were to go bankrupt or otherwise disappear without fulfilling its obligations to customers. They would all charge back their payments, and with the merchant gone, the bank underwriting the merchant account is on the hook for all that money. If they start out at $10k a month in revenue, that's $60k in risk for the bank.
If a company has no track record, poor or no credit, or a business model that doesn't fit well into a known risk model, the bank can mitigate the risk that it'll lose money by establishing a reserve account. It might be a fixed reserve (i.e. $3000 for a new company with low expected initial volume), or a rolling reserve (i.e. holding back 15% of your gross sales up to some limit for an existing account that has seen a jump in its chargeback rate).
The alternative is to deny the application altogether. Now, a bit of speculation... Stripe, PayPal Pro and other 3rd-party processors that act like first-party processors are able to take more risks (like letting you start taking payments right away without a formal application and underwriting) because they've negotiated permission with their underwriting bank to process payments on behalf of their customers with only one or a few real merchant accounts. By pooling many customers on one account, and charging a premium on the fees, they can absorb more losses and even handle merchants with >1% chargeback rates without the underlying merchant account reaching those risk barriers, which is all Visa/MasterCard/Amex care about... as long as on average, their customers are running legitimate, healthy businesses.
Moreover, you generally won't get it back until several months after you stop processing, since that's when the real risk of chargebacks goes away. Depending on your contracts and agreements, the timeframe can change significantly, including having several smaller payouts.
Respecting Paymill... well we are not going to put data on the hands of the Samwer brothers right away(just in case...).
edit: typo.
What alternatives are you now considering?