It depends on your scale. I sell on my own site at a very small scale and I just made a webform that let the user select the quantity of product on their own and get the quotation and the order would be created automatically. After that the user would be provided a link to my Paypal and they'd have to manually enter the amount and the order ID on Paypal as a remark and make the payment. Then they'd have to click on the "I paid" button on their order on my website. Then I'd receive a notification on the payment, manually verify if it's paid and fulfill the order. The payment processing is done without the being technically integrated into my site.
Since my scale is small enough, even if the user forgot to enter the order ID, it'd still work because I could just map the order and payment manually by timing.
If you don't want to create your own order handling website, even a Google Form would work for that. Just get the user to pay you on their own and fill in a form. Then you'd do the payment mapping manually and send them the product.
