\documentclass[14pt,a4paper]{letter}

\usepackage[absolute]{textpos}
\usepackage[polish]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[noprintbarcodes,nocapaddress]{envlab}
\usepackage{fontspec}
\linespread{1.3}


\SetLabel{105mm}{48mm}{0mm}{0mm}{0mm}{2}{6}
\makelabels
\begin{document}
    \FirstLabel{1}{1}
    \printreturnlabels{1}{%
        \large
        \vspace{15mm}
        \hspace{10mm}
        \vbox{\textbf{\Large Firma\\Pan Grzegrzółka}\\ul. Średnia\\01-111 Warszawa}}

\end{document}