Absolutely, he can test the functions of the very same tablet that he's reading the product information off of! It's maybe a little weird to writing directly with his finger rather than a writing utensil, but the illusion of the tablet's surface is tactile as well as visual, it feels just like the paper he bought back at Jewel's, crisp and clean.
As he signs his Arcane Mark, he'll brush against a bit of the linguistic information he received from Echo, and knows how to write his first name in the local script (the local language having a direct parallel to his name, conveniently enough), as well as an approximation of his last name.
As he's testing the tablet, a distinctly eye-catching character walks down the stairs at the back of the store. An enormous man, easily over 7 feet tall and built like a brick house, bare-chested and pale-skinned except that every inch of skin is covered with lines of fine blue script, with a shaved scalp and long, bushy mustache, he walks up to the pay counter, the clerk there quickly coming to attention and putting on a fake smile as he places a very large tablet and two small ones (that only seem smaller when held in his hands) on the counter. "I am purchasing these." He says, voice as deep as one would expect and with a thick accent.
"Th-that will be 122 ducats, sir," the clerk replies with half-hidden anxiety.
The man retrieves a billfold from his pants pocket, pays, and picks his tablets back up, all with odd slowness and sense of gravitas. "Thank you. Good day."