Zencart or Checkout to sell images and some questions

I have found 2 systems which seem to be considerable to use for selling images with Gallery2: Zencart, respectively the bridge to it and Checkout.

Checkout I have installed (it went very easily), Zencart seems to be a bit more complicated to install and cannot be deleted after installing easily, if I understand it right, so I didn`t install it until now. Checkout seems to have all, what I need as I see while experimenting a bit, so I guess I will choose it.

What are the differences between Zencart, the advantages, disadvantages?

Can I use any pdf-file for the invoices with the same file name (invoice_master.pdf), so create a new one for the invoice module or is there any information in the invoice_master.pdf which must be kept? So, as I mean to see I can use any pdf file created by myself. But how can I modify the position of the automatically added customers’ information like the address, prices: it overwrites some data of the new created pdf file. And how can I display information of my invoice, e.g. address, which should display below (and some above) the automatically added information according to the space needed by the automatically added information when downloading / automatically created the invoice so that any text will be overlapped?

Can the customer download an invoice by himself may be as often he likes to? So, is there a button automatically shown after ordering or something like that, which enables him to download his own invoice, instead of doing it by the administrator (I saw the button to do so)?