A semi-rigid holder, most commonly sold as Card Saver 1, is a single sheet of flexible plastic folded around a card and taped shut, with a lip of extra space at the top. It's thinner than a toploader and doesn't fully rigidify the card the way a hard plastic shell does.
Why PSA, BGS, CGC and SGC all prefer it
Every major grading company requires or strongly prefers cards submitted in a semi-rigid holder instead of a toploader. The reason is on their end, not yours: a grader has to physically remove hundreds of cards a day to inspect and slab them, and a semi-rigid's flexible profile and grading lip let a card slide out without edge friction. A toploader's rigid double-frame makes that same motion a place where corners catch and edges chip, at scale, across thousands of submissions a week.
Sizing
Standard 2.5" x 3.5" cards use the larger submission size, about 3 5/16" x 4 7/8" with a half-inch grading lip. That extra space isn't waste, it's exactly what gives a grader room to pull the card without touching the edges.
How it fits into shipping
Penny sleeve first, semi-rigid holder around that, taped shut, then into a team bag for the trip. This is the PSA-recommended stack, and it's different from how a card sits in day-to-day storage, where a toploader is still the right call.
Shop Card Saver 1 on Amazon → (paid link) See the full PSA checklist