According to your own links, Sezzle reports if you get Sezzle-Up and does not if you don't. It is unclear to me if Sezzle Up costs anything that the normal Sezzle doesn't. I did search for that answer.
I'm not seeing the benefit to using Sezzle, or Affirm either. Wouldn't it be better to save up for my purchase in an HYSA (giving me a nice APY) and then use a credit card from Capital One or even Bread to make the purchase (giving me a nice cashback reward), paying the credit card off from the HYSA (giving me a nice payment history and avoiding that APR)?
Unless my score is absolutely rotten, I should be able to get a secured card, a real one. And if not, a debit card is better than the service fees and stated APR.
The key is to never buy something if I don't already have the money budgeted and saved up for before I buy.
They state that there is no interest, but there might be service fees, IF the merchant accepts Sezzle. Otherwise, it says
"Pay later loans are originated by WebBank or Sezzle. Refer to your loan agreement for lender information. For example, for a $300 loan, you would make one $75 down payment today, then three $75 payments every two weeks for a 35.40% annual percentage rate (APR) and a total of payments of $305.99 which includes a $5.99 Service Fee (finance charge) charged at loan origination. Service fees vary and can range from $0 to $5.99 depending on the purchase price and Sezzle product. Actual fees are reflected in checkout."
35.4% APR... PLUS fees... At least, that's how I read the verbiage. And I have to have a subscription service to avoid SOME fees, but not all.
I'm also uneasy with Sezzle sending people to Credit Karma.
So, using Sezzle Up will report. I would go nowhere near any Sezzle at all. I'd rather use a Self card, even. At least it's Visa.
