Your percentage of markup on the hardware is based upon your decision of what you feel is fair and the other end-user prices out there. You first need to add on your hard expense such as shipping driving to pick it up , time to order etc. then decide how much you want to make that you feel is fair to the client.
Then you should absolutely factor in the cost of carrying charge on your card. You don't put "10% interest" on the invoice but when you add your profit to the hardware, that is a cost factor as well as any shipping or expense to go an pick it up. That potential interest charge is a cost and needs to be considered. You can also create a hardware only invoice with a prompt pay discount whereas you give a discount for paying quickly on the hardware and you would not incur the interest charge such as 5% 10 Net 30. Meaning if they pay you in 10 days you give them a 5% discount and if they pay after 10 days then they pay the full amount of the invoice because you will be carrying the cost on your card. The 5% represents the costs to you from your credit card company.
For example but adjust for yourself as needed.
$ 200 markup
$ 20 shipping
$ 120 Tax
$ 50 potential interest on card
Terms 3% 10 Net 30
If they pay you within 10 days they will pay you $1542.30 which gives you time to pay the credit card without an interest charge. But you need separate hardware and labor invoices
So you are covered in your costs if your credit card bill becomes due before you are paid. If they pay you before hand, you can deduct those costs from the invoice.
I disagree that the interest payments are your fault, they are an expense and not a reflection of your financial decisions. If you paid cash for the products your money is not able to make money elsewhere, that is a carrying cost. If the company does not want to incur additional expenses they could purchase the hardware themselves. With you purchasing the hardware, you make a few extra bucks