Ask HN: SaaS dealing with large enterprise orders

We are a SaaS start-up, just experiencing our first sales. Most sales so far have been small a few hundred to a few thousand dollars. We are priced based on usage.

A big customer wants us to sign an agreement for an 'order confirmation and referenced agreement' as they expect to use between $30-$100k in the first month and a recurring $5-10k/month after that.

Do companies pay this large a sum on credit cards? Or should we be doing direct billing?

Our costs are not inconsiderable on a per-use basis, so we are going to have to cover the carry for a few weeks until we get paid.

Any advice on others who have done it before? And what is important to include in the Order Confirmation.

I've looked for how to write order confirmations online, but it really all seems to be focused on small contracts.

Who/where is the best place to get advice from. This is the 3rd time we've worked with this major brand (different parties each time) but this is the largest order by a fair margin.