I haven't had any issues with Card Savers staying flat. I use a combination of Ultra Pro, Stor-Safe, and Card Saver II. I don't really have any preference between the three. I only use them for T206's so maybe with other size cards, they don't stay as flat. Here's a picture with three stacks. Left to right, 100, 200, 70. No issues of them wanting to tip over. I mainly like this type of holder because it's thin and they don't take up much space in a box.
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Ron - Uncle Nacki
T206 Master Monster Front/Back Set Collector - www.youtube.com/unclenacki
T206 Basic "The Monster" Set
514/524
T206 Advanced "Master Monster" Front/Back Set
??
??/5258
COMPLETE T206 BACK SUBSETS
Old Mill Southern Leagues - Black Ink
48/48
Sweet Caporal 350-460 Factory 30 Full Color "No Prints"
28/28
NEAR COMPLETE T206 BACK SUBSETS
Polar Bear
245/250
Sovereign 460
50/52
Sweet Caporal 150 Factory 649 Overprint
31/34
Piedmont 350 "Elite 11"
9/11