E-Pack is an online store and trading community. You can buy packs, boxes and cases of a several (but not all) UD sets. They do S1/S2, SP Game Used, OPC Premier etc. among others. They also have sets that are only offered on E-Pack (eg. Compendium).
Most base cards are digital cards only. However, you can combine several digital cards together to "upgrade" it to a different tier (eg. combine 25 base cards to 1 rainbow foil card).
Most if not all insert cards are "transferable" ie not just digital. These range from inserts, #'d base cards, autos etc. Practically everything but base cards.
Any transferable card can be sent to a linked account at COMC, or shipped directly to you.
The online community offers a chat and trade forum. Just a head's up that not everyone is honest and fair there and the chat is sometimes very toxic. If you have any other questions let me know!